Firstly do not rely on the little gauge in the dashboard, its not a reliable measurement instrument, its rather for your entertainment only as the wiring to the gauge cluster, the grouding point of the cluster, the pressure transducer itself can be squirrely (they have been garbage since 15 years now - and the ones toyota install at the factory are of a different quality/build than the genuine toyota part you buy). I have faced a lot of garbage transducers in toyotas/mazdas/mercedes benz etc.
Test the oil pressure with a proper mechanical oil pressure test rig, if you are losing too much pressure and dropping to below safe threshold then you have a leak in the hydraulic circuits, the typical oil pressure test is read as,
pressure high when cold, normalized when warmed up is an engine with good n tight bearings, lower oil pressure at cold start with a really low oil pressure when warm is a sign of worn bearings. high when cold and high when warm is a bad relief valve in the oil pump usually.
If you are hearing startup noise accompanying the lower oil pressure symptom then you have a suspect oil pump, it can be tested by revving up the engine.
The test is done with "specified" oil in the service manual.
another problem, a diesel will start to burn oil if the injection timing is off by a couple of degrees, this causes the combustion pressure to not load up the compression rings. - This is why the timing is adjusted with a dial gauge screwed in the back of the injection pump (on a mechanical pump diesel).
The timing is also affected by injection pressure in the nozzles being off - a low pressure nozzle acts as advanced timing (but with bad combustion if volume is same) - a higher pressure nozzle acts retarded. Imbalanced nozzles will make the cylinders fight each other and in some cases can cause crankshaft failure.
Do not let idiots with tools work on your engine, fix it properly, Toyota have published a service manual for it with the correct specifications, all you have to do is follow them. - This is not possible with "ustaad shaagird type idiots with tools"
