I just bought a 86 firebird trans am as a project car. It has a carberated 305 edolbrrock 1406. First thing I did was change the oil 5qts of 5w30 and oil filter. The new oil filter i put on was slightly smaller then the previous one. I started the engine and it idles at around 50- 55 psi when cold. I let it run and it went down to around 40 45 when warmed up but shoots right back to 60 when I step on the gas. I dont recall what it was at previously before the oil change i want to say it was sitting at 30 35 but the car was sitting for a while when I bought it and wasnt touched (barn find). Any help or advice would be greatly appreciated.

Factory gauge?

Try replacing the sending unit.

If that doesnt work, try popping off the intake manifold and valve covers and cleaning up all the engine sludge you find. drain oil, clean out what you can by hand. then soak lifter valley and top of heads in kerosene and continue to clean. drain kerosene periodically, but let it sit down in other parts of engine to help soak and wash out gunk. keep using new kerosene.. if you're really ambition, and highly recommend remove lifters (number each one and put back where it was) so you can clean all that out too and not gunk up the lifters. maybe even soak those in a solution to clean them up too. when pulling out rods, remember hole and direction they went in. put everything back as it was. give you a chance to inspect the top end of your engine and put new gaskets in, they are probably needed anyway. You'll likely find that your exhaust valve seals are totally gone.

then when everything back together fill with oil, drain to get most of sludge out that is down there that may remain. drain oil again. refil with new oil. start car and warm engine oil up. drain and repeat a few more times. dont forget your filters along the way. more or less a very good engine bath. its a bit pricey on oil, kerosene and filters, but it feels good. Or you could use one of the cans of stuff designed to work with oil. but I doubt they work as good as the manual method.

Yep just factory gauge thinking might buy an after market gauge too just to see if it follows

Thanks for the advice it definetly needs a good cleaning anyway!