by 'drive train 30 hyd oil ' I'm guessing you mean some sort of UTF oil. if so.. it's fine.
if the lift is strong.. but merely slow, then do this test.
with a load lifted, shut off tractor.. does it leak down ( pretty fast? )
at high rpm does it pick up speed?
hve you done a redneck flow test?
chain 3pt lift down, or put heavy implement on it.
remove plug on 3x5 cover under seat, screw a fitting with a nipple in there with a piece of hose on it aimed into a bucket. mark the bucket for 1g. start tractor and time it to fill 1 g in the bucket. can't go too much farther as sump only nominally has 2 in it.
lift 3pt handle then start....
figure your gpm.
at max throttle... you could make about 4gpm when the system was new....
that's a gallon in 15 seconds.