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  • Writer's pictureJohn Baker

Motorcycle Oil, Primary Oil & Transmission Fluid: What’s the Difference?

Some V-twin motorcycles, like modern Indian* and Victory* bikes, use a shared sump, meaning they use the same lubricant in the engine, transmission and primary chaincase. Most Harley-Davidson* motorcycles, however, use a separate sump for each lubricant. This presents Harley owners with a choice: Use the same lubricant in all three areas of the bike, or use a separate lubricant formulated and labeled for each area?

Here, we offer guidance for deciding what’s right for you.

For the record, AMSOIL recommends AMSOIL 20W-50 Synthetic V-Twin Motorcycle Oil in the engine, transmission and primary chaincase on most Harleys (consult our Motorcycle Product Guide for specific recommendations). It offers…

  • Convenience. Riders buy and install one lubricant. This reduces cost and results in fewer half-used bottles of oil lying around the garage.

  • Simplicity. Remembering to buy one lubricant is far easier than remembering three.

  • Great all-around performance in all three areas of the bike.

We formulate 20W-50 Synthetic V-Twin Motorcycle Oil to be an excellent all-around lubricant. It delivers outstanding engine protection due to its proven ability to fight wear, reduce heat, maintain cleanliness and prevent corrosion during storage.

Synthetic V-Twin Motorcycle Oil also boasts a shear-stable formulation. It resists viscosity loss despite the intense pressure and churning action of high-rpm transmission gears, allowing it to deliver reliable transmission protection.

Synthetic V-Twin Motorcycle Oil meets JASO MA/MA2 standards and is wet-clutch compatible for excellent performance in the primary chaincase. Its frictional properties are dialed-in to allow the clutch plates to engage and disengage without loading or slipping for smooth shifts.

Can I Trust One Motorcycle Oil in all Three Areas of My Bike?

Despite these benefits, some riders question the practice of using one lubricant in all three areas of their bike. They have a difficult time accepting that a motor oil can also protect the transmission and primary chaincase.

What are the Differences Between Motorcycle Oil, Primary Oil & Transmission Fluid

In essence, Synthetic V-Twin Transmission Fluid and Synthetic V-Twin Primary Fluid are formulated to protect just one area of your bike rather than three. This specificity allows AMSOIL to “over-engineer” each lubricant for its precise application.

Synthetic V-Twin Transmission Fluid has a higher viscosity than 20W-50 Synthetic V-Twin Motorcycle Oil. A higher-viscosity, or “thicker,” lubricant provides some advantage in helping quiet noisy transmission gears and enabling smooth shifts.

The lubricant develops a slightly thicker fluid film on gears, which provides extra cushion to help diminish loud “thunks” and gear noise.

Likewise, AMSOIL Synthetic V-Twin Primary Fluid is “over-engineered” to protect primary chaincase components. Its viscosity is like an SAE 50 motor oil. Formulating it as a straight-weight lubricant naturally offers an advantage in shear stability over multi-viscosity lubricants. This helps the fluid remain thicker, which helps it cling to the compensator without being “flung off” as easily for maximum protection.

In Harleys, the compensator acts as a shock absorber to prevent engine vibration from affecting the transmission. Compensator wear often leads to a knocking or ticking noise. Synthetic V-Twin Primary Fluid also clings well to the chain for excellent wear protection. And its wet-clutch-compatible formulation meets JASO MA/MA2 requirements.

So, One Oil or All Three?

So, which is the better route for Harley owners? It depends on what you value.

For riders who desire the simplicity and convenience of using one lubricant for all three areas, following our primary recommendation of 20W-50 Synthetic V-Twin Motorcycle Oil in all three areas is the best choice.

For riders who prefer lubricants dialed-in for each area of their bikes and don’t mind a few extra bottles of oil lying around the garage, use our full line of V-twin lubricants.

Either way, you can’t go wrong.

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