I'm fixin to change the fluid and double checked it (beyond the "smell test").  The weight of the fluid is much lighter than gear oil, and the color is a very light golden clear, but there still is a bit of sulphury smell like gear oil.  Maybe the previous owner topped it up with a little 80/90?  Anyway, what is in there is about to be gone.  

If you had your "druthers", would you use 10W30/40 (gas engine) or the 10/20 Universal Hydraulic fluid?  I have both (as well as lots of Rotella!).