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The oil light attaches to a pressure sensor on the side of the block pretty much right behind the exhaust manifold downpipe (the heat is hard on the wiring and it is often cracked). On an 1800 it is hooked to a 'T' screwed into the block that it shares with the oil pressure gauge flex line. A properly operating oil pressure switch will turn on at around 5 psi. A warm to hot engine with a few thousand (OK, lots of thousands) may drop the pressure under that at idle. This isn't particularly a cause for concern, just a very distant early warning tha a rebuild may become necessary 50,000 miles later. You could try slightly thicker oil, but that isn't a great solution beyond a certain point as it interferes with the oil's ability to circulate properly throughout the engine. As long as the pressure rises as the engine RPM rises I wouldn't worry about it.