20/50 oil for a pinto anything thinner is too thin. This is cheep so just change the oil and filter.
Oil pressure switch normally operate around 8 to 12 psi.
Thin oil in a warm engine will always cause the light to flicker at tickover, As long as you have the correct oil level low pressure not necessarily a problem.
I had a morris 1000 that I drove for 80,000 miles with the oil light flickering when hot at tickover.