I should learn to actually READ the entire message. Sounds like you have determined the fluid to be diesel fuel. Hopefully, it's just a bad piece of fuel line that needs either replacement or tightening of a hose clamp.

Re: the differential, I'm not sure how much the diff. on your car holds; but, it should be listed in the owner's manual. If you change it, BE SURE that you loosen the FILL plug FIRST, before draining the fluid. It's really hard to get the fluid back in if the fill plug is stuck. If your car is similar to mine, the plug is a 14 mm hex (which came with the original tool kit on my '72 220d). The volume is probably something on the order of 1.5 to 2 liters.

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Jeff Naumann
Torrance, California
1972 220D 138k miles (sold)
1982 300SD 263k miles
1989 BMW 750iL 183k miles
1993 Dodge D250 Cummins 5.9 202k Miles