I get this guestion 5 times a day...and I wish there was a straight answer for all but there is not. Some of the older machines called for motor oil or Bobcat Black gold oil. Bobcat has since started changing all manuals to read Black gold. Heres how we handle it at my place of biz,
If you need to top off, say 1 maybe 2 gal, and dont have black gold available, use the motor oil, Reason...some oil is better than no oil. Motor oil is much more refined, than say AW32 or lower, AW46 is a closer match to black gold but still not perfect. Taz stated this correctly, the plates need the bobcat fluid or accelerated wear can occur. AW46 in 2 gal or less is fine also, its just after you have diffenent fluids in different attachments, hose leaks, etc you can sure dilute the mix in your machine, and with many different people adding fluids at different times you may not have a clue whats really in your machine, Rental units come to mind here.
I would say if you have had a leak for a while and have dulited the mix over time, do yourself a favor and change out everything at your next service, your money is better spent on the right fluids rather than the new drive motors and pumps. I wished I had all the technical b/s to give you but alas Im only a dumb parts guy! My life is spent trying to find parts for machines with out a vin number, or having the guy who states he has a CASE X75 Bobcat...or the other favorite of ours the Ultra rare John-holland, and not to be confused with the brother of that machine the New-Deere. Its no wonder I look 80 years old and havent even broke 50 yet.
Mark