Yea the hydro oil tanks are where the drive chains and gears are immersed.
The bold suggests a requirement for a GL4 'speced' lubricants, such as :
a)conventional Manual Transmission Fluid, or
b)HDEO (which is more shear stable than PCMO's or multigrades) oils, or
c)Universal Transmission Tractor Fluids or
d)possibly some Universal Hydraulic Fluid (that claims being suitable for use in gear trains),
...... all of which above mentioned, are suitable for your hydraulic applications , and hence replacing the need for dedicated Hydraulic Oils (which in itself, without Oil company expressed claim ) is not quite suitable for the gears and chains system .
Stay away from Universal Hydraulic Fluids or dedicated Hydraulic Fluids that does NOT claim as suitable in gear trains applications ,which typically never carry a GL4 claim.
Lately the little thing has been running hot after 15 min and losing pressure from 115psi down to about 60 when it starts to get weak. I changed the oil in the tanks with new 10w40 but it didn't help.
At similar work load or output, the oil that runs hot after typically 5 - 8 hours (in your case, 15 minutes) of operation is broadly due to (unacceptable) inadequate operating viscosity
Too low an operating viscosity is likely caused by 'too high' an operating temperature than necessary/acceptable, particularly resulting from the use of 'too low' an oil viscosity grade specifically its value of [email protected]
*C ( not quite [email protected]
Besides, a higher operating Coefficient of Friction (due to lack of friction 'reducing' additives within the oil) leads to a higher increase of oil temperature .
So on a lark I dumped a half gallon of MotorKote in the tanks and it runs cooler now, we rarely run it longer than a half hour but I was moving hay with it for about 90 min the other day and the pressure stayed right on 110 +/- a few # the whole time and the tanks stayed cool.
I do not know anything about MotorKote , but would speculate nonetheless it did NOT significantly thicken the oil, to such a degree sufficient to noticeably reduce operating temperature of the oil in this application of yours .
I believe this cooler running oil is being helped by MotorKote's friction reducing additives
like Mo/Boron/Ti/AW etc .
Reduced Coefficient of Friction,likely in your context, leads to reduced generation of frictional heat/loss.
IMHO, a thick GL4 would be adequate to replace MotorKote's role .
So... musta been some old age friction going on in the pumps or something. If it was ready to die I figured there was nothing to lose right? It seems to have worked fine. The drive chains seem to work quieter and smoother now too.
I believe the cooler running oil is not substantially related to pump and its performance, not saying pump has no role in heat reduction.
Instead inadequate operating viscosity promotes pump leakage, which is a source of heat generation in the oil.
Edit:Viscosity-related fluid drag friction or base oil-related traction coefficient is irrelevant in your 'problem' .It's about Minimum Oil Film Thickness (MOFT) .