Do not listen to this. 10W30 and 75W90 are nowhere near the same in viscosity. Set a bottle of each outside at 20 below and try pouring them out. You'll see what I mean.
Check out the viscosities here:
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil_1_10W-30.aspx
http://www.mobil.com/Austria-English/Lubes/PDS/GLXXENCVLMOMobil_Synthetic_Gear_Oil_75W-90.aspx
I will admit my numbers are off a little but the point i was trying to get across was the same, a 75W motor oil is not the same viscosity as 75W gear oil.
We've all heard the usual terms about the "weight" of the oils. 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 trade-offs 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
On the mobil website if you would take note of the pour point point for each of the two products they are within 3 degrees of each other.
