Cadillac Repair: 1995 Deville No Oil Pressure, low oil pressure, pressure relief valve

Question
My father has a 1995 Deville with the Northstar engine.  Engine light keeps coming on.  Mechanic said there is low oil pressure.  Saw a technical paper from GM that said put 10-12 additional quarts of oil in and run up to 3.500 RPM to flood the engine and "prime the oil pump".  They tried - still no oil pressure.  Any suggestions or recommendations?

Answer
Sounds like the mechanic is looking at what he should be. Those engines have a track record of two main items that cause a low/no oil pressure condition:

1. Debris jamming up the pressure relief valve.
2. Loose harmonic balancer bolt.

The relief valve problem is what they were trying to correct by installing the extra oil and flooding the oil pump (not the whole engine), but in your case it didn't work. I'll assume they've verified the condition with a mechanical gage, and also checked the torque on the balancer bolt and found it to be within spec? If so, that only leaves internal failures as a cause in my mind.
1. Pump failure
2. Pick-up tube clogged
3. Spun bearing(s)
My guess is they will be looking to pull the engine if they haven't already, since there's not much you can do with it installed.
You didn't mention how many miles are on the engine. Maybe you should be looking for an engine instead of trouble-shooting the current one.