IMT gear oil

I am attempting to help out a neighbor with a IMT 560 Deluxe.He had placed this tractor in storage for about 5-6 years.When he pulled it out last fall
he found he had bad oil leak in the control box for the Massey 236 front end loader.He had that repaired and then attempted to move some round bales
of hay.The tractor worked fine for 3-4 bales of hay and then it started to sag when the started to lift a bale and it would require him to race the
engine to pick it up.Shortly after it would not pick up the bale at all.The lift will raise without any weight.
I attempted to do some research with this forum and found a post regarding the same issue.Within that topic I saw where the type of oil used could
make a difference.The fellow that responded to the poster stated that since this was an older tractor he should use 80/90 GL-5 oil in that tractor.The
fellow I am attempting to help is an oldtimer and he wants to sell this tractor.He does not want the put alot of money in it just to sell it.I can
understand his concerns.When he had the leaks repaired,the fellow who did the work most likely did not use the older type of oil.Could this be the
cause off these issues?Or is there something else I can look at that would cause this problem. Any guess's as to the value of this machine? It appears
to be in good condition other than this issue.

My first guess would be a cylinder piston seal has failed. It is leaking oil by the seal, causing the cylinder to not extend or hold position.

To check, take the return lines (rod end of cylinder) from the cylinder off. Gently try to raise the loader. If oil comes out the cylinder fitting, the piston seal is bad.

I should have added that this is also the case with the arms on the 3 point hitch.It will also do the same.It is not just the front end loader.The lift ,tilt and the 3 point hitch all do the same.