I had that engine many years ago- was very smooth and powerful for its size, but very troublesome too towards 100k. with oil issues.
The correct oil spec for it tho i think is 5w30 so I'm a bit curious why you using one so heavey at 20W50...can't be good for start ups. They say most engine wear happens at start up so thinner viscosity is better. You also don't say if your oil is synthetic or mineral etc. I hope you're using fully synthetic.
But it's doubtful that any of these things could be bringing on that oil light, but who knows.
I'd change the oil pressure switch first cos it's cheap, and see. Then use correct viscosity oil, and synthetic. Also we don't know what oil pump you're using brand wise and whether it is a correct OE fit so check that too. If all else fails you might have to strip inspect and re-do...hope not:dunnoanymore: