In my manual it says if you have low oil pressure check these things:
External leaks - Inspect for possible leakage.Wipe the oil lines and fittings clean,run the engine at fast idle and look for traces of oil at the points cleaned.
Oil pressure relief valve - Inspect for a broken spring or distorted oil pressure relief valve spring. Check plunger for dirt,nicks or burrs.
Oil pressure gauge - Inspect the oil pressure gauge for accuracy,by connecting an accurate gauge in line with a "T" fitting.
Oil - Make certain that oil of the recommended weight is used in the crankcase.
Excessive bearing clearance- Check the flow of oil at the bearings by using a bearing leak detector.
Oil pump - Inspect the oil pump parts for excessive clearance.