Does anyone know what psi my truck should be producing?

My aftermarket guage is reading 40PSI upon cold start.
5PSI at warm idle, and 20psi at (warn)wide open throttle.

This worries me a little, most engines i've built have been cheve's and they ushally idle around 40PSI warm.

1986 F150 302V8

 

The rule of thumb is 10 psi for every 1000 RPMs. Your pressure seems way to low. Could be excessive main bearing tolerances, excessive wear to the oil pump, wrong oil weight, etc.

I would avoid driving it till you identify the problem.