You are ahead of me Ken, I'm used to the final drive being separate from the hydro transmission. Hydraulics use hydraulic fluid, which has always been a light oil in my experience so far - usually a form of automotive auto trans fluid. I'd be surprised if you should use 90 grade diff oil (which is just about always hypoid oil, or EP90) in a hydraulic pump - I would expect that to end very badly indeed. I'm used to there being a hydraulic circuit, and a pool of oil in the final drive, but they are quite separate. I suggest you look into this carefully before you put 90 grade oil in a hydraulic circuit. Incidentally built-in tractor hydraulics often use engine oil, which is slightly heavier than ATF, but I suspect that was partly because the designers knew very well that the farmers were going to put engine oil in the system whatever they recommended, so they designed the system to cope.
