oil light on/flickering!?

Uh, oily foam in the radiator "flubber" is never a good sign. Generally somewhere between a blown head gasket or a blown engine. Keep running it and it WILL be a blown engine.

Oh, and if hot coolant is getting into the crankcase of a typical high mile slant, it will shed enough sludge to clog the pickup screen in an amazingly short period of time. See blown engine above.