The oil pressure is fine when i start it up around, 20 psi at idle. After about 20 mins of driving the oil pressure drops down to 10psi at idle. What do you guys think is causing this? pump, bearings?

 

Likely the motor is getting some miles on it and the bearings are wearing.

What oil weight are you using?

 

mobil 0w40

 

Well before I would be concerned, I would put a mechanical gauge on the car and make sure the gauge on the dash is correct. Sometimes the senders fail.

Technically speaking, by GM specs that is an Ok oil pressure (10 psi), but really on the low side of things. You could try going to mobile 15w-50 and see how that does, but its just a cover up more or less for a tired motor.