Best Transmission Oil For Harley 6 Speed: Heavy Duty trans fluid Review

Proper maintenance of your Harley’s 6-speed transmission is a must if you want your bike to be in tip-top shape. And no maintenance of the transmission can be complete without changing the transmission oil from time to time.

Changing the transmission oil will save you a lot of money you would have otherwise spent in the repair shop. Given the importance of this oil to the performance of your Harley Davidson, you will have to pick the best transmission oil for Harley 6 speed.

So which is the best transmission fluid for Harley 6 speed? Well, this is the question we seek to answer in this article. We tested a wide variety of transmission fluids and narrowed them down to six. 

Best Transmission Oil For Harley 6 Speed

In addition to reviewing the eight best transmission fluids for the Harley Davidson, we have also included a buying guide on selecting the best. Our top pick is the Spectro Oil R.HDPG6 Heavy Duty Platinum Full Synthetic 6spd Trans Lube... * (#ad).

This is one of the pricier options out there. However, it offers exceptional performance for its cost. In second place is the Red Line 90226 Powersport Motor Oil V-Twin 20W50 Powerpack Kit * (#ad). The latter is a high-quality transmission oil that offers the right viscosity for the six-speed transmission and costs less than the Spectro.

Why Should You Trust Our Review?

We crafted this review for the Harley 6 speed transmission. Therefore, we tested the different fluids to see how they performed on different Harley Davidson models. We then compiled our findings and narrowed them down to 6 transmission fluids ideal for the six-speed transmission.

How We Picked The Best Transmission Fluids?

In our selection, we considered several key things, among them viscosity. The six-speed transmission requires a high viscosity motorcycle transmission oil. Also, we considered the impact of each fluid on the gear noise. 

What Is Transmission Fluid?

The Harley Davidson 6 speed transmission allows for improved acceleration. And since the bikes are air-cooled, they heat up quickly, especially when traveling at high speeds for an extended period. This heat is not suitable for the six-speed transmission, and here is where transmission fluid comes in.

Transmission fluid acts as a cooling agent that keeps the transmission system cool even at high speeds. 

How Often Should I Change The Transmission Fluid On My Harley Davidson?

The answer to this question depends on how often you ride your bike. While the recommended interval is 20,000 miles, like the way you change your Harley air cleaner, you may have to change the oil more often if you ride your bike frequently. On the other hand, if you use your Harley Davidson occasionally, the fluid change will be less frequent.

What Kind Of Transmission Oil Can I Use On My Harley

There are various types of transmission oils, with the best to use being synthetic oil. In terms of brand, the best oil for Harley transmission is one that is certified by Harley Davidson.

How To Change Transmission Oil In A Harley

Changing the transmission fluid in a Harley Davidson is relatively easy. There are several tools you will need for this project. These are; a torque wrench, a drain pan, a new O-ring for the drain plug, a ⅝ inch socket with a socket wrench, a ⅜ inch ball-end Allen wrench, a funnel, and the fluid.

First, you will need to remove the transmission drain plug located between the shock absorbers and the transmission. Once removed, check the drain plug for any damage and remove any debris.

Take the new O-ring and slide it into the transmission plug. Once the new O-ring is in place, use your ⅝ inch socket and an extension to reinstall the drain plug. After reinstalling the plug, it is time now to add the fluid.

Use the ⅜ inch ball end Allen wrench to remove the transmission cover and dipstick. Use the long funnel to pour the transmission fluid. It is essential to know how much transmission fluid your bike requires. Do not put more than the required amount.

Spectro Oil R.HDPG6 Heavy Duty Platinum Full Synthetic 6spd Trans Lube 1 Quart

The title of the best transmission oil for Harley Davidson has to go to the Specto oil HDPG6 heavy-duty synthetic trans lube. While this oil is a bit pricier compared to other options, it is worth every penny you spend on it.

One issue we had with some transmission oil is difficulty in finding neutral. However, this is not a problem we encountered with this lubricant. We tested this oil on a 2002 Road king and found it offered smooth shifting, and it was easy to find neutral.

Also, the bike was quieter. It is perfect for the noisy 6-speed transmission of the Harley. So if you do not like the noise produced by your bike’s transmission, this oil will be the best opinion. It also features special additives that offer additional anti-wear protection and higher film strength between the gears. 

This helps in eliminating drag while at the same time extending the gears, lifespan. While it is a great option, this oil is quite expensive. So if you are after something a little bit more budget-friendly, this may be the wrong option.

Red Line 90226 Big Twin 20w50 PowerPack Oil Package

This oil kit from Red Line is a cost-effective option, especially if you live in areas with a warmer climate. So what exactly is included in this package? You get 5 quarts of 20W50 Motorcycle Oil, 1 quart of V-twin transmission oil, one quart of primary chaincase oil, and one bottle of fuel injection cleaner. 

In terms of performance, we noticed a significant reduction in the notorious shifting clunk with this oil. More importantly, the oil manages to maintain its viscosity despite its low temperatures. This makes it ideal for long-distance riding and for a heavy load.

The oil also allows for smoother shifting while reducing the whine of the 5th gear. Furthermore, it eliminates clunks. In addition to the oils, you also get a versatile funnel as well as a hat. It is also easy to find neutral, which makes it easy to find oil leaks.

Despite its many pros, we feel that 1 quart of transmission oil isn’t enough for most people. Most bikes require 1.4 quarts of primary fluid, and thus you may have to purchase an additional quart of primary fluid.

Maxima Racing Oils 90-119016C Chrome Maxima 90-119016C Twin Cam Synthetic 20W-50 Chrome Filter Complete Oil Change Kit

According to Maxima, the 90-119016c twin-cam synthetic oil change kit is compatible with Harleys of 1999 to present with twin-cam engines. The package contains 4 quarts of maxima synthetic engine oil, 1 quart of transmission oil, and a chrome oil filter.

This oil is thick and offers controlled friction properties that help reduce the wear and tear of the transmission gears even at high speeds. As such you should expect a quieter transmission. It also works well for wholesome oil changes.

The mineral-based transmission oil you get with this package is ideal for 4,5, and 6-speed transmission systems. It also works well with high-performance transmission systems.

However, with only 1 quart of transmission oil, it is about 4 ounces less than most bikes’ Harley 6 speed transmission oil capacity. As such, you may have to buy an additional quart of primary oil for it to be enough for your bike.

Bel-Ray VTwin Primary Chaincase Lubricant – 1L. 96920-BT1

Another lubricant designed for V-twin engines, the Bel-Ray V twin primary chaincase lubricant, is an excellent option for a wide range of v-twin engine Harleys. You get a lifetime warranty, 

One thing we noted about this oil is that it allows for smoother clutch engagement. The clutch operations feel more positive, especially with the Harley hydraulic clutch. This oil will also help in eliminating transmission noise.

Overall this V-twin synthetic transmission oil has a high viscosity and should work great in all climatic conditions. Its high viscosity also means it does a great job of cooling the engine. Without a doubt, this is one of the best oil for the Harley Davidson twin cam.

Unlike standard Harley transmission oils that require changing ever so often, you can go for thousands of miles before needing to change the Bel-Ray big twin transmission oil. It will be a good option for the six-speed cruise drive transmission.

Red Line RED42804 V-Twin Transmission Oil, 1 Quart, 1 Pack

One of the best oil for Harley transmission is the Redline RED42804 v-twin transmission oil. There is a lot we liked about this transmission oil. One of them was how easy it was to find neutral.

On the 2002 Road King, it slipped right into neutral on every try. Something else we also loved about this 42804 V-Twin transmission oil was how quiet the transmission system is. So quiet is it that you can hear the valve train clicking away at high speeds.

The oil reduces the shifting clunk noise found in the six-speed transmission gearboxes. Additionally, it helps keep the temperatures in the gearboxes low. Its thicker oil films between the gear teeth helping prolong gear life.

But there are some negatives that were a bit disappointing. The first and perhaps the most notable is the burnt oil smell. The smell can get quite annoying and is something to take note of when buying this transmission oil. Overall this oil is ideal for sports bikes as well.

Mobil (112812-4PK) Delvac Synthetic Transmission Fluid 

Mobil is famous for its high-quality transmission oils. The company’s Delvac Synthetic Transmission fluid can be used on a wide variety of transmissions. The oil can be used on six-speed Harley Davidson transmissions.

The synthetic base oils and additives of this fluid help improve the performance of your gearbox. Furthermore, this oil is much thicker than most. As such, it helps prevent wear and tear and thus improves the lifespan of the gears and synchronizers.

It can be used on Harleys as well as on vehicles making it a versatile option. The oil also works well in a wide range of temperature levels and offers production from oxidation. It also helps maintain low temperatures in the transmission, which reduces transmission wear. 

Overall this oil offers exceptional performance. Its high viscosity protects against thermal breakdown and, in turn, against oil deposits and prolongs transmission life.

How To Choose The Best transmission oil for Harley 6 speed

So what are some of the things you should consider when looking to buy transmission oil for your Harley? There are several things to consider, the main ones being.

Oil Viscosity

Viscosity refers to how thick the transmission fluid is. Thicker fluids are considered to be high-viscosity fluids. A thicker fluid helps prevent surface tension, which helps in cooling the transmission at high temperatures.

Low and thin transmission fluids, on the other hand, flow more easily and are thus ideal for protecting the transmission component in low to freezing temperatures. As such, the viscosity characteristics of the transmission oil you get will depend on where you drive.

In warmer climates, a high viscosity fluid is recommended. On the other hand, a low viscosity fluid tends to function better in low temperatures. For the Harley’s six-speed transmission, high viscosity fluids are recommended.

Types of transmission oil

Transmission fluids can be classified into two broad categories, manual and automatic transmission fluids. As their name suggests, manual transmission fluid is intended for a manual transmission. In contrast, automatic fluid is designed for automatic transmission.

The Harley 6 speed transmission is a manual transmission system. As such, you cannot use automatic transmission fluid on it, only manual. Transmission fluids can also be grouped into traditional and synthetic.

Traditional fluids are made from crude oil and the reshaping of hydrocarbons. Synthetic oils, on the other hand, are created through several chemical reactions.

Synthetic fluids do not thin out quickly in extreme heat or oxidize. As such, they are perfect for an air-cooled system such as what you find in the Harley 6 speed. As such, synthetic fluids are better suited for the Harley.


Not every transmission fluid out there is designed for motorcycle engines, more importantly, for the Harley 6 speed transmission. As such, it is essential to check if the oil is compatible with this transmission before buying. Using the wrong type of oil in your bike will damage your engine. Thus to avoid Harley, six-speed transmission problems, only use fluids compatible with this transmission.


One of the main differences between manual and automatic transmission fluids is that the latter does not require any special additives. The former, on the other hand, involves several additives to improve efficiency. 

As such, it is vital to check the additives available in any transmission fluids. These additives help prevent sipping and rough idling.

The cost

Transmission fluids vary in cost. Some fluids are pricier than others, and others are cheaper. Going for a cheap option is not always the best option, though. More often than not, the best transmission oil for Harley 6 speed tends to be pricey.

But you can get good options at pocket-friendly prices. Ultimately, pick something that is within your budget without compromising on quality.

Frequently Asked Questions

How Many Quarts Of Primary Fluid Does A Harley Davidson Take?

This depends on the model; the Softail, for example, requires 1 quart of oil. On the other hand, the Road King will take 1.4 quarts of oil. Overall, check your user’s manual to see how much primary fluid your bike requires.

Can I use automatic transmission fluid in my Harley’s primary?

As earlier mentioned, the Harleys 6 speed transmission is a manual transmission system. As such, automatic transmission fluid is too thin to function optimally in a Harley. It is thus not recommended to use automatic transmission fluid.

Can I use 20w50 in my Harley transmission?

Yes, in fact, this is the recommended viscosity range for most Harley’s. As such, it is the right type of fluid to use on your bike.

Can I use 75w90 in my Harley transmission?

Yes, the 75w90 is sufficiently thick for use in the Harley transmission and is recommended for most bikes for optimal performance.

What does a three-oil change mean?

This is when you change all the essential fluids in the powertrain. These include the engine oil, transmission fluid, and primary fluid.

When should I use a synthetic transmission oil on a Harley 6-Speed?

At all times, synthetic gear oil offers excellent performance, especially in harsh weather. However, this type of fluid is suitable in all weather conditions.

Is synthetic oil better than traditional oil?

Yes, synthetic oil will require fewer oil changes. Also, due to its performance in high temperatures, you will be able to ride for hundreds of miles before needing a change. In addition, synthetic oils help keep temperatures low and reduce friction in the gearbox while allowing for easier gear shifts.

Are there times when synthetic fluids should not be used?

Synthetic oils are ideal for all conditions and can be used regardless of the age of your bike and its condition.

Does synthetic oil void my warranty?

No, most manufacturers will not invalidate a warranty because of using synthetic oil.

Which synthetic oil should I use?

There are many different synthetic oil brands out there. Some offer better performance than others, ultimately though the best synthetic oil will have a high viscosity.


The transmission of your bike plays a vital role in its performance. As such, the transmission needs proper maintenance. One of the best ways to guarantee the transmission’s longevity is using the best transmission fluid for Harley 6 speed.

There are many different transmission oils out there. However, not all are compatible with the Harley’s six-speed transmission. Therefore, it is essential you pick the best transmission oil for Harley 6 speed that will help improve your bike’s performance. 

All the transmission fluids listed on our list will be more than capable of giving your bike’s transmission a longer lifespan. 

Last Updated on June 23, 2024 by Rifen

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