CM 871 low oil pressure - where to start?

[Hammerhead]

Sorry, I got interrupted while starting this reply post. I started with the oil pressure part first and see that everyone else beat me to the punch on the blowby issue so I'll leave this post as only referring to the oil pressure part of the question.

Stock tune or @m*m^2?
First, the simplest of things...oil filter. But if you don't cut it open to see if it's plugged you won't know unless you go driving again, and if that wasn't the problem, your still potentially driving with low oil pressure...
Second, the sending unit tells the ECM oil psi, the ECM tells the cab module, so if Insite tells you low pressure numbers, you still don't know for sure...
Third, if Insite tell you higher psi than the gauge reads, your gauge is most likely hooped...
If Insite and the gauge are the same, then try the sending unit...post 5 of this thread shows their location on an 871 http://rawze.com/forums/showthread.php?tid=1532
If these don't produce results, and you haven't sprung a noticeable leak from the rear flywheel housing (it contains a oil rifle) you're looking internally.
Are the rod and main bearings original?
Overhead, does there appear to be sufficient oil flow inside the rocker cover while its idling?
Verifying and answering this gives a direction of where to go next if these don't rectify it.