I'd try 1-2 PSI at most. The problem will be getting all the potential leak points in the crankcase plugged- - - -oil filler, PCV valve plumbing, etc. Putting smoke into the cylinder and watching an oil fill point might be more effective. Find a buddy who likes to vape, and let him huff some smoke into the spark plug hole! LOL! You could also loosen up the rocker arms to close all the intake valves and apply a little bit of pressure to the carburetor inlet, maybe with the blower side of a shop vacuum cleaner, or maybe a hair dryer with the heat turned off, and check for leakage at the oil fill cap.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!