For BAD Diaphragm/Seal on the Lift Pump.

Only scenario that makes sense,if the Alternate/Additional Fluid in the Sump Is Diesel.

Jeremy's explanation nails it:
"True -- the lift pump is lubricated by engine oil via the IP. If the lift pump has a bad internal seal, fuel could be pumped into the IP and then drain into the crankcase. "

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'84 300SD sold
124.128