We have run into several cases where hyd. oil is right on the verge of triggering the warning light for temp. In each case when everything else checked OK we ran a voltage check on Master Valve and found 8-9 V. The source of this was faulty Auto Header Height Control Module. This partial engagement of the master valve causes intermittant heating problems. Especially the non seal-pac'd old open type module that was SUPPOSED to have been changed out. If you have this symptom try this check. The second leading cause of hot oil is a leaky RWA motor that is bypassing case drain oil to reservoir. Of course the obvious should be checked first but these are a couple of things that we overlooked that were hard to find but are showing up more as the age of machines increase. Keep in mind that if you have an old HHC module that you can see the diodes and transistors you have a module that is not powered by the engagement of the sep. and could be a safety problem. As always be SAFE!!!!