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3M advesive, tar, and wax remover. And then something like Meguiars paint cleaner. Then some wax.
Yep the only thing I've seen work is exactly that - the 3M Bug and Tar remover. From there you could try some Zymol HD Cleanse (essentially a paint cleaner.)
YMMV. This did wonders on my buddy's red 951. My black 951 cleaned up a lot but still doesn't look too hot. I'm thinking I might just go and have the rockers repainted (along with new stone guards on the rear fenders.)