Oil weight for an older engine

I'd been putting 10w-30 into my engine for the past 25 years. The engine is a 400ci SBC from a mid-70s GM truck. Not clear if it ever been rebuilt, as there were no records when I bought the car. I suspect not - it was more likely picked from a junkyard, hosed down, and painted to match the car.

I read a recent magazine article by somebody resurrecting an older hot rod (a barn find). Part of the process was draining the oil and filling the engine with 20w-50. Have I been doing this wrong? Are the older engines designed for the heavier weight oil? The oil pressure has always been low, in the 30-35psi range.

Thanks for help and opinions.

Dorsey

I see no reason to change. Keep doin' what you're doin'. And there is nothing wrong with 30-35 psi as long as you aren't drag racing and buzzing the motor to the moon.
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Times 2, 30-35 psi is perfect for a 25 plus year old motor. Don't change a thing.
Many guys think the higher number / thicker oil is better, but that isn't actually true for a street car.
In many cases, the "thicker" oil actually causes stress / strain on motor components like oil pump drives etc. ,
You've been doing it right for 25 + years - keep up the good work!

Thanks, all for calming me down. It's a low-stress engine in a car (pictured) used to run errands and cruise the hot spots at low speed.

I think you should cruise over here and take me for a ride!!! rotflmao! 8-)

Engines need oil flow and pressure is a characteristic of flow and viscosity. If you increase your viscosity unnecessarily, you could end up causing more flow to recirculate back to the sump through the relief valve. I would use a 10W-30 heavy duty engine oil (I use Petro Canada Duron SHP 10W-30) in my engine with a dual rating (ie, API CK-4/SN) that also meets Ford F1WSS‐M2C171‐F1 (for more ZDDP).

Your donor truck's owner's manual would likely have called for 40-grade oils in summer temperatures. You can easily use any 15W-40 heavy duty engine oil and this grade's higher HTHS viscosity will give your bearing more protection. See Corvair Oil Guide and Engine Wear.