CM 871 low oil pressure - where to start?

[RSCTRANS]

2010 international lone star with Cummins CM 871. Earlier today I noticed my oil pressure gauge drop down to about 25 psi under load. 504,000 miles on the engine. Within the last six months I have been reading and performing all of the maintenance on my motor, that I have learned on this site. I did a forced regen a week ago and the differential pressure reading came back at .50. I was very happy. I have even went as far as buying an auxiliary power unit and had it installed two weeks ago. This way I do not idle my engine at night. Six months ago there was a crank case pressure issue. I put in a new air compressor, that did not take care of the problem. Then I replaced the turbo. That seems to have corrected the issue at that time. However, about an hour ago my check engine light came on and then it went into shutdown mode shortly after. I carry the Cummins insite program with me. It once again read high crank case pressure. I took the shortcut for today so I could make it home about two hours away, and drilled a quarter inch hole in the crankcase filter cover facing the driver on the top. However, I am concerned that I have something else going on. Where should I start? Oil filter? Gauge? Sensor? Oil pump? Or could it be internal? Please advise and any suggestions are greatly appreciated. Thank you in advance. As a sidenote – I have a boost gauge that is reading between 25 and 30 psi when turbo is kicking in pulling the hills. I have a 10 speed transmission with 373 rears. That's how the truck came to me when I purchased it used with 300,000 miles on it. I have been trying to keep it around 1450 rpm's to 1500 rpm's for better fuel consumption. Again, any advice is greatly appreciated.