Chevrolet Camaro 1967-2002 - 20w50 oil for LT1?

I hadda mechanic recommend I put Castrol GTX 20w50 in my LT1 with 147k miles. If it makes a difference I have headers and an air intake system on it. I hadda do an oil change anyways so I used his recommendation but with the viscosity I'm running at 70psi oil. Highest it'll go in acceleration is like 75. I'd been always running 10w30 and that always kept me at a comfortable 40psi. I see online people say you can and can't use it. I thought giving I have 147k miles I'd be okay but 70psi is making me uncomfortable. Should I switch back? I'd hate to make the engine work any harder.

I would never use that in mine , And if you live up north keep in mind winter is coming and that oil will be thick as hell when it gets cold . I have almost the same miles as you with a cam etc. and i use 10w30 and have no problems at all with that . just my 2 cents

I used 5w30 Mobil 1 full synthetic in my car with 167xxx and my dad in his non-rebuilt 226xxx mile LT1 same oil. I ran the piss outta his car and it begged for more. I'd say 5w30 is fine. No ticks or knocks at all. Quiet as a mouse.