Hydraulic oil for models - what's in it?

[fhhhstix]

My dad and I had a little 4wd loader tractor given to us some years ago by his boss as he did not use it any more and it was hard to get parts for so he did not want to sell it to us so he gave it to us and said good luck. The tractor ran great it was a SUZUE 150 a 15 horse diesel 4wd with a loader, 3 point hitch and PTO. We used it the first summer no problem then came winter and I decided to use it to plow the drive way and to my surprise the loader would not work so I backed it from the barn to the garage and let it run while I went to find the manual. A 1/2 hour later I tried the loader again and it had started to work but slowly I looked it over and noticed that the line going to the hyd pump was connected to the transmission. The person who designed this decided to use the oil from the Trans to run the hydraulics and the Trans uses 80/90 gear oil. So in the middle of a snow storm I changed the oil to the only thing I had in the garage that was not 80/90 I use 15/40 diesel oil and it has worked great ever since. Apparently wherever this tractor was built it never gets below 90 degrees.