I went to look at a 98 trans am ls1 auto 96k miles today
It was warm and when idling only had 20 psi oil pressure on the gauge. (stock of course)

It got a 40 psi pretty easily.
I didnt really get on it either.
He said he was running some synthetic oil in it also.

Do those sound like decent readings?
Would thin oil cause this?

 

If you get on the throttle a little, it will go over 40 psi.
It has 40 psi normal driving.
Let off the gas completely and it drops to 20 psi.

normal? common problem? thin oil?
oil is not low, and its nice and clean.

 

Sounds good to me.

 

Yep sounds normal to me too.

 

I run 0w-30 Syntec (the good Euro blend that everyone raves about) and usually idle at or just above 40psi. At WOT, I probably see upwards of 70psi thanks to the ported oil pump that was installed with the H/C setup. Stock pumps should see close to 60psi @ WOT. When the oil really gets warm, you may even see it dip down close to 25~30psi at idle.