I have an 89 d250 5.9 cummins and I check the oil regulary and it doesnt use or leak any oil out but every couple weeks my oil pressure at idle is low and my oil pressure light will come on but go off when i am driving it. What could be the cause of this?

 

Before you go chasing down low oil pressure gremlins, check the actual pressure at the test port next to the turbo oil feed line on top of the cooler (where the oil filter screws on). The oil pressure switches on these trucks are well known to be unreliable and inaccurate pieces of garbage. Mine has awesome oil pressure but the factory gauge tells me I have none at idle and very little at WOT.