A reader asks: “I’m looking to switch my Harley-Davidson Street Glide Special over to AMSOIL. Obviously the engine will take your synthetic 20W-50, but I’m curious about what to use in the primary and transmission. There are a lot of different combinations put out there on the internet. Some use gear lube in the transmission and chaincase. Some even use ATF in chaincase and claim they have great results. What is the best combination for my bike?”

Answer: For any Harley Twin-Cam engine, we highly recommend using AMSOIL Synthetic 20W-50 in “all three holes” (as they put it) for two essential reasons.

1. It Works (and works very well indeed)

AMSOIL 20W-50 Synthetic Motorcycle Oil is specifically designed to withstand the rigors of engine, transmission and primary chaincase use. Expect smooth, quiet shifting with maximum component life. It is contains additives to prevent wear and the 100% synthetic base oil will not shear down (thin out), leaving transmission or primary components vulnerable to accelerated wear.

AMSOIL has secondary recommendations of AMSOIL Severe Gear 75W-110 for the transmission and Severe Gear 75W-140 for the primary, but the company themselves will tell you that there is absolutely no upside to using gear oil in these components rather than the 20W-50. Some may think that they are using a thicker oil by using a gear lube, but remember that engine oils and gear lubes use different viscosity scales to prevent confusion.

Let’s look at the actual viscosity numbers of AMSOIL 20W-50 Motorcycle Oil and AMSOIL Severe Gear 75W-90, 75W-110 and 75W-140. The industry standard for measuring viscosity is in “centistokes” at 100°C.

  • AMSOIL 20W-50: 20.4
  • Severe Gear 75W-90: 16.8
  • Severe Gear 75W-110: 21.3
  • Severe Gear 75W-140: 27.5

AMSOIL Synthetic 20W-50 Motorcycle OilAs you can see, the 20W-50 motorcycle oil is very close to the 75W-110 in terms of actual viscosity, so those opting for a gear lube thinking that they are using thicker oil, really are not. Those using an 80W or 75W-90 in their transmissions are actually using a much thinner oil. The 20W-50 motorcycle oil’s viscosity is right in the “sweet spot” for optimum operation in all three holes and the volumes of positive reviews we receive back this up.

Gear lubes do contain heavy doses of “extreme pressure” additives which many believe prevent transmission wear and provide quieter operation. Again, AMSOIL 20W-50 Motorcycle Oil is packed with additives specifically designed to prevent wear in clutches, gears, chains and sprockets, so again there is no downside to going the 20W-50 route.

As for the use of automatic transmission fluid (ATF) in Harley primary chaincases, we have heard that some riders do this, but we do not recommend this option. Again, let’s look at actual viscosity numbers to back this up. The viscosity of a typical ATF at 100°C is 7.5 centistokes. We submit that ATF viscosity is far too thin for this application and think that the risk of long term wear is far too great. Again the AMSOIL 20W-50 has a viscosity of 20.4 centistokes and is designed for chaincase service. ATF is not designed for this type of service and frankly we don’t think that using it in chaincases is a good idea.

2. Convenience: One Effective Product for Everything

The second main reason that we recommend using 20W-50 motorcycle oil in the engine, transmission and primary chaincase is that you can buy one product for everything. You can keep one product on hand and bring one oil along on trips for top-ups. Not only does AMSOIL Synthetic 20W-50 work exceptionally well in all three holes, it keeps things simple.