1992 400SE

I was driving yesterday when the oil level light came on. The light would go out when I slowed down to idling speed, and would turn on again at higher speed. When I got home there was a puddle under the car and the oil level was down about 2 quarts. The oil seems to be leaking from somewhere under the oil filter housing. My first suspect was oil cooler hoses, as they are shown on the service CD diagram, but looking from the top I can't see any oil cooler pipes or hoses attached to the bottom of the oil filter housing.

Was oil cooler optional on this car?