oil light on/flickering!?

How much of a sludge bomb is this engine?

You "got" your oil changed?!? You mean you didn't do it? Stop not doing this yourself.

First, replace the oil light switch. They get gummy after a while.

Then, if that doesn't fix it clean the relief valve. If a bit of crud blocks it open you'll have low pressure at low RPM.

Something else to consider with old slants, especially if the sat for a while, didn't have their oil changed regularly, or mostly saw stop and go (granny) driving is that oil pressure at the switch does NOT mean oil flow at the rockers.