On the filter shaft is a tiny hole. Blow some compressed air through it to make sure it is not clogged. If you still have the belly pan under the motor ( the noise encapsulation panel ) it will have to be removed to get to the drain plug. Or you can use a topsider and suck the old oil out of the dipstick hole. I prefer to drain it when the oil is hot.

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John
1986 300sdl
1985 380se