Sae 90 gear oil is equivalent to a sae 50 oil.
Not exactly. It's more equivalent to something around an SAE40 as a baseline.
Sae 90 gear oil is equivalent to a sae 50 oil. So this being sae 60, shouldn't be much thicker.
SAE 50 TO-4 oil viscosity @ 40 DegC is about 195 cSt. The same oil in an SAE 60 is 340 cSt @ 40 DegC - almost double. The figures below are for Mobil but all other brands will be within less than a couple of % of these number.
Guy at cat dealer was saying they use 85w-140 in the compact track loaders. How does that compare in thickness?
The difference in the 85W-140 is that it's a multigrade and when it's cold it will behave like an 85W oil, or somewhat akin to an SAE 30. Multigrade is an option that should be considered when running into the performance limitations of a high-viscosity oil during the critical cold starup & warm-up phases of machine operation.
Cat are talking about FDAO being factory-fill on machines that not only are many orders of magnitude larger than your track loader but also run 24/7 and therefore are rarely subject to cold starts like your machine will be every working day. As I mentioned before
FDAO has some major caveats regarding operation on cold machine startup. I've worked with dealers and on job sites that have used FDAO since it was first introduced back somewhere around 2004. Believe me when I tell you that IMHO it's not the right oil for your final drives.
As a matter of interest what type & brand of oil are you using right now.?