Check how hot the oil cooler actually is, just by feel. If it is cool, then Gpurr is probably right on the ball. If it is very hot then there will be a flow restriction further upstream. I have had a couple of cases [ not on Gleaners ] where restrictions ie. bits of hydraulic hose rubber, partially blocked hoses and caused real problems as well as being a total pain in the back side to find. This plugging can also occur in valves. It means a slow, tedious and systematic going through the system until you find the restriction. In Australia we have gone to an AGCO reccommended heavier hydraulic oil spec due to our high air temps and heating and oil break down problems. Probably not much help but good luck. Cheers!