Low oil pressure at 1500rpm

[Rawze]

34 - 36 ~ish is normal at 1500 rpm unless the oil temp is high (230+ degrees).

What you describe sounds low. -- On the ISX, low oil pressure is most of the time caused by oil filters getting clogged due to excess soot problems in the engine. If the oil pressure comes back when changing filters out then you need to solve the excess soot problems and also do a few oil changes at 5,000 miles intervals to help flush out old soot buildup.

- Older engine also loose oil pressure when the rod/main bearings start wearing out and when the cam bearings in the head wear out. 800k miles (or equivalent engine hours) is the recommendation for rod/main bearings, so if that has not been done, that should be done as well.

it can also be the oil pump wearing out.

On an engine with that many miles on it, using a gallon of 80-90 gear lube or maybe even 85-140 if it has a lot of wear in it at each oil change would help extend its life a bit and improve oil pressure too.