2010 KW T660 with Cummins ISX with 98k miles and about 19k miles on the oil.
Just recently I have noticed the oil pressure slowly dropping from 35-38 pounds down to 21-25 pounds over the course of about 45 minutes to an hour of steady highway driving. If I stop and shut the truck off for 2-3 minutes and start it back up I get the oil pressure back to 35-38 pounds and start the whole process over again.
I also notice that if I am climbing hills while it is at the low pressure end it will drop a little more and then, when the fan kicks on to cool the water, it will regain a few pounds but never gets above 28 pounds pressure.
One more piece of info....the oil temp will remain between 225 and 230ish while the water temp remains in the 180 degree range other than when climbing hills.
I'm thinking oil pressure sensor. What do y'all think?