I had a leak inside the oil guage, if you pull it you'll be able to tell if it's the guage or not pretty easily. There is a flat banana-shaped copper tube inside there that's crimped and soldered on both ends. Mine was leaking at one of the crimped ends, and there was oil all over the tube and inside the guage. There was actually some telltale oil on the outside of the guage as well, but not much. That oil will trail across the cluster via wicking action and drip wherever it feels like dripping. Some say you can silver solder it and cure the leak, but getting to the leak site can be a real problem. New oil/temp/fuel guages can cost from $375-$600! I got a used eBay cluster and now all is well.

It really can only be those two things- the vacuum shutoff/ignition ass'y or the oil pressure guage. Or else somebody is hiding inside the dash, dripping dirty oil on your leg.

Be sure to use a 13mm backup on the 10mm guage line fitting to keep from damaging it when removing/installing

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)