The Definitive Guide for Best Transmission Oil for Harley 6 Speed

Harley 6-speed engines can give you an immense amount of power, and you can get the best motorcycle riding experience. However, for that, you also need to change your gearbox transmission oil now and then. In fact, you need to get the best transmission oil for Harley 6 Speed engine.

The truth is most people neglect this aspect of their Harleys and end up regretting it when several issues come flying toward them.

Because we care for you and want your bike to run perfectly, today we are presenting to you the 5 top transmission oils for Harley 6-speed engines out there!

Top 5 Picks of Transmission Oil for Harley 6 Speed

What Weight Oil for Harley Transmission?

A lot of debate goes around the topic of the right weight oil for Harley transmission, and in this part of our article, we would like to shed some light on it. Transmission oil that is 20-50W is often seen as ideal for Harley transmission because, in terms of viscosity, it hits the exact right spot.

The viscosity of your transmission oil plays a crucial role in determining how your motorcycle will run, so you will have to ensure it is right. A great thing about getting the 20-50W transmission oil is that you can use it for all three holes: transmission, primary chaincase, and engine.

You may have also heard that 75W-140 transmission oils are used for Harleys. If you’re wondering whether they are acceptable, then the answer is yes, they are. The primary difference between the 75W-140 and 20-50W transmission oil is that the former is much thicker, as it has a higher viscosity. The thicker fluid has advantages, such as it makes the motor quieter, but the disadvantage is that the motor has some difficulty.

Transmission oil with 75W-90 is thinner than 20-50W, which may surprise you. So, it is also an acceptable option as a transmission oil for your Harley. The only issue is it can be a bit too thin at times, which makes the engine susceptible to long-term wear.

Harley 6 Speed Transmission Oil Weight

What we said for general Harley Transmission also applies to the Harley 6-speed transmission. Although we can’t exactly guarantee which weight of transmission oil will be the best for your Harley 6 speed transmission, we’ll say that the 20-50W and 75-140W are some good picks.

5 Best Transmission Oil for Harley 6 Speed Reviews 2022

Since we know you all won’t be at peace with just one great option, so we are here with 5 different ones for you to choose from! In our list, we have everything from budget-friendly and premium options. So, let’s look at what makes these transmission oils so great.

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

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

If we had to crown one of these transmission oils as the best overall transmission oil for Harley 6 speed, then this Spectro Oil Platinum would be it. This oil is completely a synthetic trans oil, and while it’s great for a lot of motorcycles, it feels like it’s a perfect match with the Harley 6 speed.

With an oil grade of 75W-140, you won’t need to worry about the viscosity of the oil and having a jammed transmission. Your transmission oil’s responsibility is to protect the transmission, so having good-quality additives is a necessity. Thankfully, this transmission oil uses high-end G4 additives to protect the engine from such damage.

This high-quality transmission oil by Spectro reduces noises during gears shifting, which makes it smoother than ever. The reason for this is that the oil has the perfect oil grade.

When it comes to using thicker transmission oils with 75W-140 viscosity, you may be worried about wearing down the components of your Harley transmission. However, with Spectro Platinum Oil, this is no issue. Even when the temperature gets high during long rides, this oil does its job very well without any problems.


  • Amount: 1 Quart
  • Type: Synthetic
  • Viscosity: 75W-140


  • Gear shifting is smooth, and the transmission doesn’t make noises
  • Operates well at high temperatures
  • High-quality additives help in improving the strength of the film
  • Gearlife is increased by G4 technology


  • Costs more than the average transmission oil

2. Red Line Oil Big Twin 20W50 PowerPack

Red Line Oil Big Twin 20W50 Power Pack

By buying this package, you will be getting a WHOLE powerpack. Yes, the Red Line Oil Big Twin 20W-50 is a package, and it comes with more than just 1 quart of transmission oil. You get 5 quarts of motorcycle oil, 1 quart of V-twin transmission oil, 1 quart of the primary chaincase oil, 1 bottle of fuel injection cleaner, and more.

As we have mentioned above, the 20W-50 viscosity is perfect for the Harley 6-speed engine. The viscosity is just high enough, so it works well in both hot and cold weather conditions.

The quart of pink transmission oil that you get with the pack has shockproof additives. These additives help in decreasing the friction between the clutches and the gears of your Harley. Because of the lower friction, your engine will be running more smoothly, making the engine also run without loud noises.

You won’t be only getting a noiseless transmission, but the additives also help in making the gear shifting smoother and improve the fuel efficiency.

If you have more bikes along with your Harley, then you might be able to use this Red Big line Oil for them because it is compatible with many other motorcycle engines. The other products that come with the kit must also be highlighted because by buying this package, you’re getting everything you need for a full oil change.


  • Amount: 1 quart
  • Type: Synthetic
  • Viscosity: 20W-50


  • ­20W-50 oil grade makes it work well in hot and cold environments
  • Shockproof additives reduce friction and increase the lifetime of internal components
  • Comes as a whole kit with most of the other oils needed for a full oil change
  • Widely compatible with other bike engines


  • The only fluid it doesn’t have is additional chaincase oil

3. Maxima Racing Oils Twin Cam Synthetic 20W-50 Chrome Filter Complete Oil Change Kit

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

Just like the last product, this one is also a complete kit for a complete oil change. From the name, you can tell, this transmission oil was specially made for twin cam engines which is why it suits the Harley 6 speed engine perfectly. 

Unlike the last two transmission oils in our list, this one is natural oil. Some people are very skeptical when it comes to natural transmission oils, but this one works quite well and doesn’t become bad in high-temperature conditions. This is because it has a high viscosity of 80W-90. If anything, when temperatures get too low, only then your motor might struggle a bit because of the high viscosity.

You will be glad to hear that this transmission oil also makes shifting gears very smoothly. It has good film strength and is great at lowering shock and friction between the gears and clutches. Thanks to the additives in the oil, you will be able to use it for a while without having to replace it.

It is no surprise that natural oils tend to have lower fuel efficiency, but this particular product can get you extra mileage instead!

Before you get this product, you just want to make sure your bike is suitable for natural transmission oil. Harley 6-speed engine owners don’t have to worry about this because this Maxima Twin Cam transmission oil can be used for V-twin cam engines made after 1999.


  • Amount: 192 ounces
  • Type: Natural
  • Viscosity: 80W-90


  • Functions well in high-temperature conditions
  • Has impressive engine cooling ability
  • Can keep the gearbox sludge free with the help of detergents
  • Can work effectively in high-pressure environments
  • Additives reduce the wearing of parts


  • High viscosity, so it gets a bit too thick in extremely low temperatures.

4. Lucas Oil Synthetic SAE 75W-140 V-T

Lucas Oil Synthetic SAE 75W-140 V-T

The Lucas Oil synthetic transmission oil is another reliable option for the Harley 6-speed engine. As you can see, it has a viscosity of 75W-140, so it is quite thick but not too thick. It can reduce the clanking transmission noise significantly, which is very important for a comfortable ride. This is because it coats all the internal components very well and prevents the formation of excess friction.

Due to its ability to effectively coat all the parts of the transmission and reduce friction, it also supports much smoother transmission operation and gear shifting.

Its cooling ability is also something that we appreciate. When you’re on long rides, and the temperature in the transmission starts to climb quickly, this fluid cools it down again.

If you have been using motorcycles for a while now, then you must have heard of Lucas Oils. This brand is very reliable, and they won’t be scamming you by making false claims, so you don’t need to worry.


  • Amount: 1 quart
  • Type: Synthetic
  • Viscosity: 75W-140


  • Great cooling ability
  • Reduces clanking noises of transmission when the bike runs
  • Decreases friction and wearing of the parts
  • Reliable brand


  • Clear oil, so difficult to notice leaks

5. Mobil (112812-4PK) Delvac Synthetic Transmission Fluid 50

Mobil (112812-4PK) Delvac Synthetic Transmission Fluid 50

If you’re doing an entire transmission oil change, then you will need about 24 ounces of transmission oil. We would recommend you to buy this Mobil Delvac Synthetic Transmission fluid because it comes as a pack of 4 at a reasonable price. Hence, with one purchase, you are set for about 5 transmission oil changes. So worth it!

On top of that, this is an SAE 50W-90 lube, which means it can function in hot conditions easily. It has high film strength and stays very stable. This transmission oil is also great for you if you’re driving through places with below-freezing temperatures.

Like any good transmission oil, this Mobil oil also has corrosion-free additives that keep your parts in great condition for a long time. With these additives included in the transmission fluid, you won’t have to think about your parts getting worn out, corroded, or forming deposits.

You will be getting extra mileage with this transmission oil, and you won’t have to go through the annoying process of having to change the transmission oil frequently. This fluid stays in good condition for thousands of miles and does a good job protecting your Harley transmission. One thing that makes this oil reliable is that it meets the industry standards for transmission oil from GL-1 to GL-4 gear protection.


  • Amount: 4 quarts
  • Type: Synthetic
  • Viscosity: SAE 50W-90


  • Very stable in both hot and cold conditions
  • Great value as it comes in a pack of 4
  • Provides excellent protection against corrosion and deposit formation
  • Compatible with more than just Harley


  • Your initial purchase cost will be higher since you’re buying 4 quarts together.

Things to Consider Before Buying Transmission Oil for Harley 6 Speed

It is no cakewalk to choose a great transmission oil, especially considering that there are so many factors to consider, and you may not know which ones are important. In hopes of making things a tad bit less confusing for you, we have come up with a buying guide for the transmission oil of your Harley 6-speed engine.

Which Type of Transmission Oil do you want to get?

There are primarily two types of transmission oils: natural and synthetic. Both types of oils can be good or bad, depending on a lot of factors. But the most important thing is you ensure whether the type of oil you’re choosing is compatible with your Harley or not. Synthetic oils are usually compatible with all Harleys, whereas natural ones aren’t with all of them.

Another thing we want to mention is if you’re considering a natural mineral-based transmission oil, then make sure it can withstand and is stable at high temperatures.

Additives Quality

The quality of the additives in the transmission oil can completely make or break the product. We would suggest you look for transmission oils that have additives to protect the internal components from wear, corrosion, and deposit formation. Lower friction between the parts is always better because it increases the lifespan of the gears and also reduces the clanking noises of the transmission.

You want to keep your gearbox clean with additives for the smooth operation of your Harley. Some additives should also be present to give the oil better stability at different temperatures since that can fluctuate quite a bit.

Oil Grade and Viscosity

Viscosity refers to the resistance that a fluid faces when it flows. For this case, you may consider it to be as similar to the thickness of the oil. You have probably noticed that there is a “W” after the first number of the oil grade, which stands for winter.

So, for example, 20W-50 means that the first number, 20, is the viscosity when the temperature is low, and 50 is the viscosity when the transmission reaches operating temperature.

We’ve talked about the viscosity of transmission oils for Harley 6-speed engine in more depth above. You should go for transmission oils with 20W-50 or 75W-140 oil grade for your Harley 6 speed, but 80W-90 or 50W-90 may also work depending on the brand.

Is it Compatible with your Motorcycle?

There is a compatibility issue when it comes to picking a transmission oil. Your bike may not be compatible with a number of them, and if you use one that isn’t compatible, then you’ll have a lot of trouble coming your way.

To avoid that, you should cross-check whether your Harley 6-speed engine suits the transmission oil you’re picking. It can be that the transmission oil has all the right specifications, but it just might not be compatible with your motorcycle.

FAQs About Harley 6 Speed Transmission Oil

1. What is the transmission fluid capacity of Harley 6 speed?

It can take about 24 ounces of transmission oil when it is empty, so that is less than 1 quart of fluid. Hence, you’ll be fine if you buy 1 quart of transmission oil.

2. Are Synthetic transmission oils better than natural ones?

Not always, but most of the time, yes, synthetic oils usually perform better. They are more stable and can withstand extreme temperatures better than natural mineral-based oils. However, there are great natural oils such as the Maxima Racing oil that we have included in our list.

3. How frequently does the transmission oil of the Harley 6-speed engine need to be changed?

It will be necessary for you to change your transmission oil every 10,000 to 20,000 miles. But we would suggest you change it every 5,000 miles just to be safe.

4. Can Synthetic Transmission Oil invalidate your Warranty?

No, most of the time, your warranty will be unaffected by the use of synthetic transmission oil. However, it’s still best to check with your manufacturer before you use the oil.


Don’t overlook the importance of good transmission oil. After you have read this detailed article, you should have a clear idea of how it can determine the overall performance of your Harley.

From the 5 options that we have presented to you, we hope you can pick the best transmission oil for Harley 6 speed. Additionally, our buying guide should be of immense help. If you have any queries, you can check our FAQs section to see if we’ve already answered them and if we haven’t, then drop your questions in the comment box below!

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