With solvents it pays to start carefully with the least likely to harm and if it doesn't work move up to the next. Always use the solvent on a piece of rag start with some washing up liquid, then WD40, Jizer, Turps (oily pine tree derivative), White Spirits, (petroleum oil derivative), nail varnish remover and finally try spray paint thinners. After that you're onto abrasives!!
edit - don't use hairdryers with solvent - Boosh!!!
Two things to clarify:
1 when I say turps I mean turpentine which is different to Turps subs (Turpentine Substitute) Turps Subs is almost the same as White Spirit.
2 by abrasives I mean starting with Brasso then cutting paste as used on cars. These will polish the paintwork rather than scratch it.