I have a technique that worked to get some very nasty 3M adhesive off paint. Dunno if it works on tires - but I think it's worth a shot.
You get a MF wax applicator pad (like the square ones at AG), then dampen it with mineral spirits. After that, add a few blobs of compound to the working area of the pad. Use the pad as a scrubber. The combo of compound abrasives and MS seem to really do a lot of removal work. Remember, compounds are like liquid sandpaper. So, you want an aggressive compound here.
I can't see compound abrasives scratching a tire too bad. I can't see a little bit of MS damaging a sidewall either. I just would clean it well with APC after.
Another possibility would be a Magic Eraser.
After you get this Turtle mess off - check out Tuff Shine. TS is removable by scrubbing the tire with their cleaner. However, you never need to remove it - because it doesn't "brown", peel, or turn to sticky goo. I've had it on tires for over a year (it has been topped off) - but tires have never been scrubbed and look good as new.
