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tl;dr - but: I had a 2008 Smart Diesel, and it was grateful for an addition of half synthetic two stroke oil, about 1:150 mix. It ran definitely quieter. MPG had been the same though. I felt better because I assumed that the high pressure common rail pump was better lubricated.
Once one guy had created his own Diesel fuel addon in a DIY way named Monzol. I had tried it, it was even better than two stroke engine oil.
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