Best Transmission Oil for Harley 6 Speed | Top 10 Picks

Did you know that the type of transmission fluid you use for the engine can heavily affect the ride quality? Well, yes, it can! The gear control degraded substantially when one of our team members skimped out on the oil. Not to mention, the ride was uncomfortable, and let us not forget the irritating noise on highway speed.

For that reason, we always recommend getting the best transmission oil for Harley 6 speed. However, most of the riders get confused with the transmission fluid quality.

But the good news is that we know the ins and outs of Harley bikes, thanks to years of riding experience. Also, we have a fair idea regarding which fluid can make the bike roar the performance it can provide. And we will use our expertise and experience to ensure that you end up picking the right oil for your Harley.

List of Harley Transmission Oil:

Best Transmission Oil for Harley 6 Speed

We have put a reasonable amount of time into testing and comparing the available options. And in our intensive tests, these are the ones that seemed worthy to us:

1. Valvoline Advanced SAE 5W-30

Valvoline Advanced SAE 5W-30

The first thing that we considered while choosing the best Harley transmission oil is the level of protection. And this offering from Valvoline excelled in that factor with flying colors.

When it comes to the oil type, this one is synthetic. And its formula can offer high overall temperature protection. As the quality of the oil is reasonably high, it can resist breakdown exceptionally well. It has a proper amount of antioxidants inside, which will fight degradation. In other words, it will last for a long time.

As the label states, the viscosity is 5W-30. Compared to other lightweight oils, this one is capable of providing better overall sheer protection. The high-quality nature of the fluid will even offer high overall mileage.

It will also enhance the lifespan of the engine. There are engine-clearing detergents in the formula. Those will fight against deposits and keep the parts of the engine clean.

On that note, 5 quarts of this oil is 3.23 pounds. Handling the bottle will be an easy task as well. It features an easy pull table and has a precision pour spout.

Highlighted Features:

  • Provides high-temperature protection
  • Highly resistant to breakdown
  • Offers high sheer protection
  • Enhances the mileage
  • Packs engine-clearing detergents

2. Castrol 159AE1

Castrol 159AE1

The brand Castrol is pretty well-known for offering excellent engine oil for the money. So, we had to check whether they had something that was a good pick for the best oil for Harley transmission and primary. Well, they did!

One of the main highlights of this Harley Davidson gear oil is the shear stability. Its viscosity is at 20W-30, and it will not break down for the high shear stability. The formula even offers high protection on the critical zones of the bike. In other words, it will enhance the lifespan of the parts.

This Harley gear oil also performs exceptionally well in high temperatures. So, the engine will not throttle that easily. It even holds the ability to improve the mileage by at least five mpg, which is pretty impressive.

That said, the formula is synthetic. And 1 quart of the fluid weighs about 2.19 pounds. So, it does fall under the lightweight category. The oil also features race-derived technology, which will ensure that the engine performs at its very peak.

Highlighted Features:

  • Has a high shear stability
  • Resistant to viscosity breakdown
  • Protects the crucial zones
  • Performs well in high temperature
  • Increases the mileage

3. Mobil 1 96936

Mobil 1 96936

While looking for a primary oil for Harley, you might want to get something that can reduce wear at extreme temperatures. In that case, consider this offering from Mobil 1.

This transmission oil for Harley 6 speeds is fully synthetic. As the quality of the formula is exceptionally high, it can offer better protection to the engine during extreme temperatures. The formula can also enhance the mileage and make the engine highly efficient.

On that note, the Harley transmission oil weight is at SAE 20W-50. The viscosity is at 20W, which will make it compatible with loads of Harley models. And it lasts longer than most of the other average options out there. So, after putting this in, you will not need to make a Harley transmission oil change anytime soon.

Apart from that, the formula has special additives that will lower sheering. The metal-to-metal contact will be minimal. That will also do a proper job of enhancing the overall lifespan of the parts.

Highlighted Features:

  • Offers better protection from engine wear
  • Enhances the overall mileage
  • Long-lasting
  • Has special additives
  • Improves the efficiency of the engine

4. ACDelco 10-9395

ACDelco 10-9395

Aeration and foam control is another factor that we considered while looking for the best oil for Harley. And this offering from ACDelco has managed to impress us in that regard.

The formula is entirely synthetic. And this synthetic formula holds the ability to control the aeration properly. As a result, the chances of foam building up inside the engine remain exceptionally low. Thanks to the high-quality nature of the oil, the mileage will also see a massive improvement.

But that is not all! The formula has high viscosity stability. For that reason, the viscosity level will not degrade that quickly. It can also resist oxidation and friction exceptionally well, which will eliminate the sheering issues.

Additionally, it also provides extended drain intervals. As the formula offers high overall temperature protection, the oil will not degrade that quickly either. So, you can expect it to last for a prolonged amount of time. And 1 gallon of the oil will weigh somewhere around 2.28 pounds.

Highlighted Features:

  • Controls aeration properly
  • Substantially improves the mileage
  • Has a high viscosity stability
  • Resistant to friction
  • Capable of resisting oxidation

5. Red Line RED42804

Red Line RED42804

Transmission noise is one of the issues that many Harley riders face over time. Well, with this oil from Red Line, that will not be an issue.

As we mentioned, the formula has the ability to smooth out the shifting. It will lower the notorious shifting clunk and dampen the overall transmission noise. As a result, the ride quality and comfort will greatly improve. It also offers high protection from high temperatures by lowering the operational temperature.

The oil viscosity and oil weight is at 75W-90. And it creates a thick oil film around the gear teeth, which will lower sheering. This film will also do a proper job of resisting throw-off and reducing the overall temperature even further. You are sure to get a smooth overall ride quality.

It is compatible with all high-revving four-stroke bikes, including loads of Harley models. Also, it integrates shockproof technology that will protect the engine parts from big impacts. The formula can also massively improve the gas mileage. And as it runs cooler, it will last for a prolonged time.

Highlighted Features:

  • Smoothes out the transmission shifting
  • Creates a thick film around the gear teeth
  • It runs at a low temperature
  • Provides proper protection to the engine parts
  • Can resist throw-off

6. Maxima Racing Oils 40-04901

Maxima Racing Oils 40-04901

On the lookout for a mineral-based oil? Well, Maxima has got the thing that you have been looking for all this time.

The formula is not synthetic. Instead, the brand has opted for a mineral-based composition. Its quality is reasonably high, and it will enhance the transmission’s overall performance. Also, the formula is compatible with the models that utilize spring clutches and wet diaphragms.

Other than that, it integrates extreme pressure additives. Those will offer better overall protection in high temperatures and pressure. The surface-active chemistry of the formula provides better protection against shock loads. Also, the chemistry will smooth out the shifting of the transmission and dampen the noise.

It has ultra-clean detergents that will protect the gears from deposits and wear. The formula can even resist mechanical sheering and is highly resistant to breakdown. So, you will not find yourself replacing the oil that often. And as the formula is mineral, it is pretty light.

Highlighted Features:

  • Mineral-based and light in weight
  • Packs extreme pressure additives
  • Enhances the overall performance
  • Protect the gears from wear and deposits
  • Resistant to breakdown

7. Bel-Ray 96900-BT1

Bel-Ray 96900-BT1

Maximizing the power by minimizing the sliding friction is something that most of the formulas are not capable of offering. But that is not the case for this offering from Bel-Ray.

The lubricity of the formula is exceptionally high. That is what makes it capable of lowering the sliding friction, which eventually enhances the power of the transmission. This synthetic oil has a viscosity and oil weight of 85W-140. Now, that is comparatively lighter than most of the average options.

Thanks to the high-quality trait of the oil, you can make it work optimally for around 10000 miles. It will not need that frequent changes. Also, it is directly compatible with the Harleys that came out in the late 90s and afterward. So, you will not have to worry about compatibility that much.

It also integrates special additives to smoothen out the transmission process. As a result, you will get a quiet overall shifting. The temperature protection is reasonably praiseworthy as well. And for its ability to lower friction, metal sheering will not be an issue either.

Highlighted Features:

  • It has a high lubricity level
  • Lowers sliding friction
  • The mileage is reasonably praiseworthy
  • Packs lubricity additives
  • Smoothens the shifting

8. BEL-RAY Big Twin

BEL-RAY Big Twin

Are you looking for something that will extend the gear life? You need to take a look at what Bel-Ray is offering here.

The synthetic oil features a special formula for motorcycles with separate transmissions. And that includes most of the bikes that are from Harley. Nonetheless, the unique formulation will provide excellent overall protection to the gear and extend the overall lifespan.

Its viscosity is at 85W-140 and has special additives to resist wearing. In other words, you will not have to worry about metal sheering that much. The oil also creates an oil film around the parts, which will smooth out the shifting and lower the gear noise. So, you can expect the ride to be exceptionally smooth.

You will be well amazed by the quality of the oil. It is highly resistant to breakdown. As a result, you will not have to go through the hassle of frequently changing the oil. It also provides good overall protection from high temperatures. That will further enhance the lifespan of the engine.

Highlighted Features:

  • Extends the overall lifespan of the engine
  • Resistant to wearing
  • Smoothens out the shifting
  • Lowers the engine noise
  • Long-lasting and resistant to breakdown

9. Heavy Synthetic Gear Oil

Heavy Synthetic Gear Oil

There are not that many good heavy-weight primary and gear oil available out there. However, if you are looking for one, keep this unit in your consideration.

First of all, it features a dual viscosity of SAE 80W-140. And this synthetic formula has a special formulation to work on most Harley models. There are special additives inside the blend. Those will increase the flow rate and improve the cold-start ability of the engine.

The additives will also play an essential role in protecting the engine from high temperatures. Its specialized lubricity meets the anti-wear requirements across a wider range of temperatures. The formula will substantially enhance the gear’s lifespan.

Furthermore, it is formulated for maintaining a proper friction coefficient for the clutch operation. That will smoothen out the shifting and lower the engine noise. You can also expect it to lower the metal sheering. The HD transmission oil also requires less frequent changing.

Highlighted Features:

  • Has a high flow rate
  • Improves the cold-start ability of the engine
  • Offers excellent protection from high temperatures
  • Lowers wearing
  • Maintains a proper friction coefficient

10. Red Line V-Twin

Red Line V-Twin

Not all of the available transmission oils can lower the operational temperature properly. And that is what exactly makes this offering from Red Line stand out.

To start with, it has a viscosity of SAE 75W-250. However, the fluid friction is much like the 75W-90. For that reason, the oil film that it will create will substantially smoothen the shifting and lower the overall noise. Your ride will feel much more comfortable after putting this on your bike.

The formula also operates at a much lower temperature. It offers high protection to the engine from high temperatures and ensures that the wearing is substantially low in the transmission. The thick oil it creates will also do a proper job of resisting throw-off.

Additionally, the lifespan of the formula is pretty praiseworthy. Thanks to that, you will not need to worry about frequently changing the transmission oil. Also, as it can lower the overall friction between the gear teeth, they will have an enhanced lifespan.

Highlighted Features:

  • Creates an effective oil film
  • Capable of smoothening out the shifting
  • Lowers the shifting noise
  • Protects the engine from high temperatures
  • Resists throw-off

What to Look for Before Buying?

The review section is not enough if you want to get the most out of your money and high overall performance out of your bike. Along with just going through the reviews, you should also keep these crucial factors in your consideration:


The first thing that you need to check is the type. Mainly, there are two types of transmission oil. The first one is synthetic, which is the most common Harley Davidson transmission fluid type. On the other hand, there are mineral-based oils.

For selecting the right type, consider the recommendation of the manufacturer. In this case, check the manual of your Harley Davidson bike. It should state the type of transmission oil that you should be using.


Another thing that will depend on the manufacturer’s recommendation is the viscosity. However, if you cannot find any recommended viscosity stated in the manual, we recommend getting the thinner ones. Those will have a high flow rate, which will enhance the engine’s overall performance.

Quality of the Oil

Quality is another thing that should be in your mind before making any purchase decision. The higher the quality, the better the overall performance will be. Also, higher-quality transmission oils can offer protection from deposits and can adequately protect the engine parts from damages.

Transmission Oil for Harley 6 Speed


You would not want to go through frequent oil changes, would you? Well, for that reason, you must keep longevity in mind. Again, the quality of the oil will play an essential role in this case. Usually, the high-quality ones will last longer and need less frequent changing.

Protection from High Temperature

Transmission gears can reach a reasonably high temperature during operation. And that high temperature can lower the lifespan of the parts. For that reason, we would highly recommend picking the oils that offer better protection from high temperatures or can lower the operational temperature.

Friction Protection

If you happen to pick a transmission oil that cannot create a proper oil film around the parts, the level of friction will be reasonably high. And when the metal-to-metal contact is relatively high, the parts will wear out faster. So, we would highly recommend picking the ones that can create an effective film around the gears.


Last but not least, consider whether the oil can increase the overall mileage or not. Some of the options can improve the mileage by at least five mpg. Now, even if five mpg might not sound like a lot, it will surely help you save quite a good chunk of money in the long run.

What Is Speed Transmission Oil?

Speed transmission oil, otherwise known as lubricants for gearboxes, plays an important role in ensuring a smooth overall ride. As the name suggests, it will lubricate all of the moving parts of the transmission, which will ensure that the gear system runs smoothly.

Transmission Oil for Harley 6 Speed

Furthermore, as these oils create an oil film, the friction level lowers substantially. That will protect the internal parts from heating and wear. So, apart from ensuring a smooth overall operation, the transmission oil also plays a crucial role in enhancing transmission’s lifespan.

Which One Is Better Synthetic Oil Or Standard Oil

Standard oil, otherwise known as mineral oil, can offer proper lubrication. The lubrication that it offers surely enhances the performance of the transmission. Mineral oils can even smoothen out the shifting and lower the operational noise.

However, mineral-based transmission oils usually do not have any additives in them. Synthetic oils have those additives, which will enhance the lubrication process. The additives also offer higher overall protection from deposits, wearing, temperature, and much more.

Transmission Oil for Harley 6 Speed

Considering that, we would say that synthetic oil is better than standard oil. In other words, synthetic oils have the edge over mineral-based oils.

Does the Use of Synthetic Oil Void My Warranty?

Not really! In fact, most of the Harley Davidson bikes will ship with synthetic transmission oil. And there is no policy that states that you cannot use synthetic oil over mineral-based or petroleum-based oils. For that reason, you can use synthetic oil without worrying about voiding the warranty.

Frequently Asked Questions

a. What oil goes in a Harley transmission?

Gear oil, speed oil, or transmission oil will go into the transmission of the Harley. However, do ensure that you are using a proper oil that can lubricate the gears and other parts properly.

b. Can I use 75w90 in my Harley transmission?

The 75W-90 will mostly be synthetic oil. And it has the right viscosity as well. So, you can use that oil without worrying one bit.

c. Harley 6 speed transmission oil capacity?

When it comes to Harley transmission oil capacity, it is around 24 ounces or 0.71 liters. Make sure not to overfill because that would cause some serious leakage.

d. How often should I change the transmission oil in my Harley?

That will highly depend on the lifespan of the oil. However, generally, you should change the oil after every 5000 miles. If the oil is high in quality, you can get away with changing it once every 10000 miles.

e. What weight oil do you use in a Harley-Davidson transmission?

This will depend on the transmission of the bike. And different models of Harley Davidson have different transmissions. Nonetheless, 75W-140 is the most common one.

f. What is the viscosity of Harley transmission oil?

The OEM transmission oil for Harley has a dual viscosity of SAE 80W-140. It is basically a heavy-weight gear oil.

g. What oil does Harley Davidson recommend?

The manufacturer recommends synthetic oil that has a viscosity of 20W-50. However, this recommendation is not fit for all of the models.

h. What oil should I use in my Harley primary?

It would be better to pick something that is 10W-40 or 20W-50 as a primary oil for Harley Davidson bikes.

Final Words

It is essential to pick the best transmission oil for Harley 6 speed. Regular oils cannot even come close to what that has to offer. And hopefully, we were capable of making the choosing process a bit more manageable for you. Also, do make sure to consider all of the critical factors before making a purchase.

Leave a Comment