I'm not sure that there's any good access point to the oil system other than at the oil pressure sending unit. While there are probably some oil galley plugs somewhere on the engine, finding one that you can actually get to is not going to be likely. Personally I don't think you've got an oil pressure problem - the 6G7x engines just don't have that problem unless they're really abused and neglected, and you sound like you've been the exact opposite of that with your maintenance.

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes