Has anybody got suggestions on the best way to remove the oily soot deposits that accumulate on the trunk lid and rear of the car, without stripping the paint finish off? My Ivory White SDL really shows up the residue and its driving me nuts trying to keep it clean. I did a search, but no conclusive answers.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown