I had to have my right rear axle replaced. The inboard CV boot was torn (arrived that way new) and it slung
oil and grease all over the place. It was definately black. As I recall, it was more like grease, but had an
oil component to it as well. The grease probably heats up and thins as you drive. Can you tighten the clamp at the boot, or is it fixed and there is nothing to do? If it throws enough of the grease out, it will dry out and the CV joint will fail. Maybe a call to a friendly BMW wrench turner can give you some ideas. My RR axle was replaced under warranty by Cal Cobra, so maybe your dealer can help.
Hank