Adding extra gallons of oil only adds mass, which only lengthens the time it takes to overheat. And I have done the calculations for heat loss on painted steel reservoirs and it isn't very good, especially if one side is being heated by the engine!
You really only have three basic options:
1) reduce the input power: most likely not a viable option.
2) reduce the flow losses in the system: quickest bang-for-the-buck is probably the quick couplers (if you give us manufacturer, part number, and flow someone can probably find the pressure drop), review and possibly increase hose sizes, review oil viscosity being used and possibly replace with lighter-weight grade (follow recommendations in manual), more difficult but effective is replacing 90° fittings with curved hose end fittings or short pieces of tubing.
3) or increase the cooling: (Looking at an online parts diagram it appears the radiator and oil cooler are side-by-side with a pusher fan. The hyd cooler doesn't look very big. Pushers generally move less air than suckers, but in this case you don't want to pull in the crud in the air and plug the radiator.) Check that the shroud is well sealed against the radiator, the bristle brush is around the fan opening and just touching the fan, and the fins aren't all bent over due to a high pressure sprayer.
There also appears to be a pressure bypass in the lower hydraulic cooler header - if it is leaking hot oil will bypass the cooler. I could recommend some tweaks to the shroud, but they won't be worth the effort unless you are just shy of having enough cooling.
If you still need more cooling the best option is a remote mounted secondary hydraulic cooler. The best location would be in series after the original. Another possibility is in the return line just after the lift arms, but the back pressure will be higher and should be checked when cold to make sure it won't blow up the cooler. Use a thermally switched fan to allow the oil to heat up and keep the fin spacing on the large side to help prevent plugging. AKG makes some killer HD units with fans and easy to install that I can vouch for, but they are on the pricey side.
ISZ
