Use any of the available car wash soaps , they all cut the Diesel residue and road grime without attacking the paint , they're cheap too .
Or , use SWMBO's " Woollite " (just don't get caught !
) as it's gentle but good at cutting oil & grease too .
If you have your Merc' polished once or twice a year , the oil and spooge will slide right off wheever you wash it.....