So ordered the tar remover from Sonax Monday and applied earlier (the application description followed). All sticker residue could be removed completely and the still super easy. The specified exposure time of 60 seconds was in most places not even of necessities - just shortly afterwards sprayed and wiped with a microfiber cloth over it. Without residues and effortlessly within minutes any sticker removed.
A truly great product that does not promise too much. NOTE: I have tested the effectiveness ONLY when adhesive residues, not for tar etc.