Which choice is better for Harley-Davidson transmissions?

A particularly nice feature about AMSOIL 20W-50 Synthetic Motorcycle Oil is that it can be used in the engine, transmission and primary chaincase of Harley-Davidson motorcycles. But some riders wonder if a gear lube like AMSOIL Severe Gear Synthetic 75W-110 or 75W-140 is a better choice. Let’s explore this question.

AMSOIL Synthetic 20W-50 Motorcycle OilFor Evolution 1340 and all Twin Cam engines, AMSOIL’s first recommendation for the engine, transmission and primary chaincase is AMSOIL 20W-50 Motorcycle Oil. For the transmission of these models, they offer a secondary recommendation of AMSOIL Severe Gear 75W-110 and a third option being Severe Gear 75W-140. Some riders claim to experience quieter operation with these options, but are there real world benefits?

First, let’s establish that AMSOIL 20W-50 Synthetic Motorcycle Oil is fortified with anti-wear additives to deliver maximum gear wear control. It is also formulated to resist shearing for long service life. The Severe Gear Lubes will work just fine, but will not offer improved wear control over the 20W-50 motorcycle oil.

Let’s point out that the AMSOIL Motorcycle 20W-50 and Severe Gear 75W-110 are identical in terms of thickness at operating temperature. Gear lubes use a different viscosity grade scale on the label to prevent confusion between gear lubes and motor oils. Here is the kinematic viscosity of each product at 100 degrees centigrade as shown in centistokes (cST).

20W-50 Motorcycle- 20.4 cST
75W-110 Severe Gear-  20.3 cST
75W-140 Severe Gear- 27.5 cST

So will a thicker oil offer better protection in Harley transmissions?

No that is not the case. The Severe Gear 75W-140 may offer quieter operation simply because of the thicker viscosity. It doesn’t necessarily mean that better wear control is the result. In fact a thicker oil could potentially result in slightly higher operating temperatures.

Bottom Line

AMSOIL 20W-50 Synthetic Motorcycle Oil aces the FZG Gear Test with a perfect score of “zero” wear and again, is loaded with anti-wear additives to deliver the ultimate in wear prevention. It is also formulated to limit friction for optimum shifting performance.  If you wish to use one of the Severe Gear products, that is no problem. The bottom line is that there is no appreciable advantage to using them over the 20W-50. The convenience of using the 20W-50 in all three holes trumps any perceived advantage in our opinion.