Same problem with my 69 Camaro wheel. I have had limited success washing the wheel in dishwasher with dish detergents (Wife not happy). I have also used thinner, and other similar products but I've never been able to keep it from coming back once it gets hot in the sun. I have also had many other GM cars with the same problem. I'm kinding thinking on the same line as badbob about oil and grime from our skin. I would think that any chemical solvent based releases from manufactering would have flashed off years ago.
At any rate this is something I've never been able to figure out.
Do other makes of cars have the same problem?