I was at an airfield that sold Avgas 100LL, Mogas and Jet-A. Although clearly airside, the Mogas pump specifically specified "not for aviation use".
Anyway, I don't think all Thielert/Centurion engine installations are approved for diesel fuel (it was a supplement to the DA40 POH after all), and from what I recall from reading the Flyer article about this, other engines (SMA?) may not be approved for car diesel ever.
Having said that, car diesel is cheaper than Jet-A, at least over here (1.09-ish vs. 1.50-ish) so if there was a choice, I'd run on diesel.