Did you just get the truck or engine? Was something changed that might have started the difference? That out of the way here's some unlikely things to do while your pondering pulling the cam...
1) Changed the filter? Maybe something goofy with it's bypass (not sure if this would cause drop in pressure though).
2) Replace the spring as Chiphead mentioned.
3) Take a sample of oil and test the temperature.

40 Chevy 1/2 ton