For no apparent reason, after my engine comes up to operating temperature (usually after about 5 minutes), the oil quantity light illuminates. When I turn the engine off and restart it, the light appears immediately and does not extinguish for the remainder of the operating cycle.

When the engine has completely cooled down, I can start it up again and the light won't illuminate until it gets back to operating temperature.

The oil quantity is fine--right at the top plastic mark where it's always been.

Any thoughts?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles