Not sure what kind of library you have down there, but here is the excerpt from my 4.236 workshop manual.
Section D - Fault Finding
Low Oil Pressure
4 - Incorrect grade of lubricating oil
36 - Worn or damaged bearings
37 - Insufficient oil in sump
38 - Inaccurate gauge
39 - Oil pump worn
40 - Pressure relief valve sticking open
42 - Broken relief valve spring
43 - Faulty suction pipe
44 - Choked oil filter
53 - Choked gear
box or engine oil cooler
58 - Blocked sump strainer
You can eliminate many of these based on the original symptoms. But then again, if someone cracked into the oil system to do major maintenance
, then you may want to physically eliminate these one by one to be sure unrelated faults haven't been introduced.
Good Luck ... Amigo!