444XT High Oil Pressure

A few details: '89 Peterbilt 379, Cummins 444XT. This is my boom truck that doesn't drive down the road very often, mostly it sits on a jobsite with the motor running to operate the crane.

I'm having a few symptoms that may or may not be related. I noticed the other day my oil pressure is starting out really high (around 90) when I first start the truck. The pressure slowly goes down to around 35 as the engine warms up. It takes awhile to get there though. As the rpms rise so does the oil pressure at about the same rate.

I'm getting oil in the intake. I noticed my manifold pressure gauge wasn't reading so I removed the air line at the manifold (it's really an aftercooler) while the engine was running. Some oil came out but not under pressure. I'm assuming the turbo is passing the oil, I think that the only place up stream of the manifold where it could come from. My intake vacuum gauge isn't working either, it's on my list.

I'd like to open up the aftercooler to clean it out but it looks like it has to be removed from the engine to do this. Anyone know any tricks to getting the cover off without the guts falling out? The thing looks pretty heavy to remove as one unit.

I plan on changing the oil and filter. The cummins filter on there now is the 3318853 dual flow filter. I'm not necessarily assuming that this is the right filter since I've found a lot of other questionable things done to this truck since buying it. The manual I have doesn't list a part number and Baldwin lists a full flow, dual flow and bypass filter options for my engine. How do I know which one to use? I know it's not the bypass filter.

Also, is there anything I could do to help the engine since in runs mostly at idle with no airflow from going down the road? I've been seriously considering finding a smaller four cylinder engine to run the crane hydraulics and leaving the big motor for driving. The oil and coolant temps are around 125.