There is a lot of re-paving going on in my neck of the woods. As a result, the insides of the wheels on my 2010 Boxster base were full of what I refer to as "tar boogers". Upon removing the rear wheels, each one had 40-50 of these things stuck to the inside. I tried Meguire's alloy wheel cleaner, which did nothing to the tar boogers, followed by RainX Bug and Tar removal gel, which was equally impotent. After this, I went old-school and broke out a $5.00 can of WD-40. I sprayed the boogers with WD-40, let it dissolve the blobs for ten minutes, then hit the blobs with an old tooth brush and hosed it all off. Success! It took a little while, but WD-40 and a little elbow grease was the solution.