FWIW, on four occasions over the past five years, I've added one gallon of used oil to a full tank of diesel on a couple of my 240Ds. One of those gallons was from the hydrostatic tranny of a friend's Kubota tractor, while the other three were from my gasser car with substatial freeway mileage. (I won't burn that black, sooty, used oil from my diesels)
Only problem I ran into one time was premature clogging of the pre-filter in my automatic 240D from some junk that had gotten in the jug I used to collect the used oil.

OTOH, last fall I added a gallon of used oil to top-off the tank of my stickshift 240D. The previous summer I had noticed pinging under part hrottle in that car, but it has dissappeared since that one tank of oil-added diesel.

I'm not that worried about an occasional gallon of used gasser-oil added to a tankfull of diesel on these cars. Most of the engine/injection problems I've heard about here seem to have resulted rom problems with waste veggie oil.

Happy Motoring, Mark

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DrDKW