OK, now we have to know if your mechanic pulled the engine and just changed the seal or if he did any other measuring, rebuilding, fixing etc. Pontiac stock clearances are a bit tighter than a lot of others from the muscle era and usually have enough pressure to lift the relief valve with about 65psi at speed. 25 psi at idle is about normal in a hot engine, 20 is a bit low but a lot of guys have around 10.
Pressure is a result of a restriction to flow, the higher the restriction the higher the pressure as long as the supply source is able to keep pressure. Most Pontiacs use a Melling 54 oil pump. It flows enough to lubricate the engine with enough pressure and flow to keep it from destroying itself. As long as the bearings, main, rod, and camshaft are tight enough to restrict the flow, the pressure will be high. Stock Pontiacs only had about 1.5 thou. clearance on the mains. If the main bearings are worn to 3 or so thou the pressure will drop. Increase in flow will result in a higher pressure than a lower flow with larger clearances, but the stock pump should be enough to keep the pressure up as long as there is not an increase in clearance somewhere. If you have an increase clearance anywhere in the oil system the pressure will drop. The only way to know if you have a clearance problem is to measure everything when the engine is apart. The oil acts as a wedge between the bearings and the journals. The rotating member spins in the film of oil and doesn't touch the bearings, theoretically anyway. I think there are a few guys on this site that would be happy to get 20 psi at idle. Mine has 25 psi at 650 rpm in 100 degree weather at a stop light. I have 2.5 thou on the mains and rod bearings, I had to buy two sets of cam bearings in order to find bearings thick enough to give me the proper clearance on the cam bearings.
Your guy will find a problem, if there is one, when he measures the bearings in the bores and the journal size, but I wouldn't get to excited about 20 psi at idle. But if you have the engine apart anyway and he finds an excessive clearance the you may as well replace the bearings, if that is what the clearance problem is. If the crank journal is just a smidge small but you still get 20 psi, I would put it back together the way it is rather than grind the crank.
