Oil Light coming on (right oil?)

[CFELD]

So it's coming to winter and I just rebuilt the m42 I am running (all new gaskets and bearings). I am running 10w-40 as of right now, and I plan on doing the first oil change this weekend. The oil light comes on when the car is at operating temperature and below 1k rpm. It also comes on sometimes when I start it. Also it make an aweful racket sometimes just after I start it (like the tappets have no oil) for anywhere from 10 seconds to a couple minutes. The oil rotor looked fine to me as well as the pressure relief valve.
Should I start by filling oil through the filter housing? Does that make a difference? I live in chicago so it's about to get cold, I was wondering what weight I would use because maybe I am not using the right weight and thats why the light is coming on. Thanks

[1998ccc]

I use 0-40 Mobil 1.  Light goes out right after start up.  Have you checked oil pressure with a gauge?  General rule is 10 psi / 1k RPMS hot.

[CFELD]

I am using 20w-50. Right now I think it is maybe the relief valve. I am getting 8psi at warm idle and  60 psi near redline.

[1998ccc]

What are your rod and main bearing clearances?