Submitted By Skip: (pococj)

We’ve all heard the usual terms about the “weight” of the oils used in our bikes. 20w-50, 75w-90, 10w-40 - all these terms are bandied about with ease for most of us. But what do they really mean?

Without getting totally scientific, the terms represent viscosity measurements. They aren’t “weights”, as we like to call ‘em. Viscosity is defined as: the property of resistance to flow in a fluid …

We say that lower “weights” are thinner, meaning they flow more easily, or have less resistance to flowing. We want both the ability to easily flow during start up, and more resistance to flow at higher temperatures. The multi-grade oils we use give us both these attributes, and help our engines live much longer than in the day of using straight 50w, 60w, and even 70w in an old Pan or Shovel motor. But there are some tradeoffs with the multi-grade oils. They “shear” more easily than straight oils. Shear is when the oil’s actual molecular chains are sliced into pieces. When that happens the viscosity drops, and the oil provides less protection.

That’s just a bit of a basic oil primer. In the past year or so HD, as well as several oil manufacturers, have started recommending the use of their 20w-50 synthetic engine oil in more than just the engine. You can use it in the primary to lube the chain, and in the transmission, where it must operate in a gear-grindin’ environment. So, how can HD say to use 20w-50 stuff where they once called for 75w-90 stuff? Wouldn’t it be too thin to do the job?

Here’s the answer: Viscosity is measured by different scales for engine oil and gear oil. The two systems aren’t directly comparable, number-for-number. Here’s a reasonably accurate comparison between the two systems. Please remember that I said “reasonably accurate”, as there is some overlap.

SAE Engine Oil = SAE Gear Oil

10w thru mid range of 20w = 75w

Mid 20w thru mid 30w = 80w

Mid 30w thru low 40w = 85w

Low 40w thru 50w = 90w

60w up = 140w


Looking at these ranges we notice that 50w engine oil is the same “real” viscosity as 90w gear oil. So it stands to reason that they might be substituted for each other. But remember that there are other factors involved than just viscosity. The ability to withstand shear, and additives designed for a particular end use are also important.

The shear aspect and the additive packages are why some of us don’t think it’s the best idea to use the 20w-50 engine oil in a transmission. There are many reports of tranny damage where the 20w-50 was used. However, there are currently no scientific studies to prove the damage was the result of the use of those oils. You must make the decision as to your own bike.

Personally, I’ll continue to use gear oil in my tranny. There are enough differences in the engine vs. tranny applications that I feel better going that route.