Oil presure on a SBC 350?

20 psi at idle in gear when hot is perfectly fine for a gen 1 sbc. You say you did an oil change - did you change to a different make oil filter or lighter weight oil? For example, I know that Fram filters give me almost 10 psi less at idle on my LT1 when compared to an AC Delco filter, and synthetic 10w30 gives me about the same when compared to standard 10w40. Flow through the filter will contribute to different psi readings and different oil pressures in the engine, but what value you are seeing really doesn't sound like anything that would make me nervous if it were my car.