The oil light in my '82 240 came on yesterday while I was driving. I stopped and checked the oil, which was full and clean. It has a Mann oil filter which is less than 3,000 miles old. Car has over 200,000 miles on it. When key is switched off and engine restarted, oil light goes off for 5 seconds or so, then back on. The light stays on strady and does not flicker or go off with increased RPMs. There is no rattling from the bottom end of the engine (which there certainly would be if an engine with that many miles had zero oil pressure), and I drove the car over 100 miles with the light on. No problems, no knocking. So I assume the oil pressure is OK. But what would make the light go off upon startup, then on after 5-15 seconds of running? Could the filter have an internal defect? Could the sending unit be bad? I've never heard of a sending unit failing and causing the light to stay on; usually when they fail, the light doesn't go on at all. What do y'all think?

TIA
Colin