The best stuff is called "Desolve-It"

Sold at Wal-mart only like $4. i use it daily when removing 10 year old decals that have built up a gunk and tarr on body and paint. it is safe for the body and not harmful to anything. it is a grease base material. best thing to do is spray it - let it settle on the "gunk" for a couple of minutes then take a flat surfaced. clean edged hand tool (ID card or something) and scrap at the "gunk" it will come off so easily. then just wipe down the area and spray the area with windex or alcohol and is a nice clean surface with no gunk or harm to your body/paint/windshield.