Specific Gravity of Diesel Fuel VS Motor Oil

Send oil in to get it tested. They will tell you if fuel diluted.

Diesel fuel is quite soluble in lube oil. If your crankcase level is increasing with no indication of water present then what you have is known as fuel dilution. It is quite common in the large marine diesels I have worked with. You need to not only change your oil but find out where the fuel is leaking into the engine. Many diesels have been ruined by fuel dilution. Usually the only indication is rising crankcase level and possibly but not necessarily a decrease in oil pressure. I'm sorry. I know this is bad and probably expensive news. Good luck. -Mark.

Motor oil and diesel fuel are mutually soluble, so very unlikely to form separate layers. Between suck-&-mix of oil pump and oil spray coming off the bearings and bottom edges of cylinder walls, you got some strong mixing going on. If you are getting diesel in the crankcase, you gotta change the oil before low lubricity ruins expensive pieces.

faucet1 wrote:
I suppose I was thinking about a cold engine and checking the dip stick when I posed the question. Crankcase seems to show above full reading since the last outing(maybe a qt). No milky substance. So, probably not antifreeze. My question is, what is it and where did it come from. Was not there before last trip.

Good time to change oil. Have you given it the sniff test??

I suppose I was thinking about a cold engine and checking the dip stick when I posed the question. Crankcase seems to show above full reading since the last outing(maybe a qt). No milky substance. So, probably not antifreeze. My question is, what is it and where did it come from. Was not there before last trip.

in a running engine, the oil pump will pull both in, mix them, and pump the now thinned mix through all the bearings. I would be more worried about melting the piston..