It is the CV grease, typically on the left rear wheel for clockwise tracks because I think that wheel gets more heat? And also the direction of the for corners forces the now liquified/overheated grease out that vent hole...
I removed both axles, removed the inner CV Joint, soaked it all in kerosene (or maybe diesel) and then rinsed it all out with acetone, and repacked with Redline CV-2 grease, put on new boots and clamps and put it all back together. 1 small tub was enough to do all 4 cv joints, and this totally resolved the issue for me
Edit: when it comes it, it is pretty liquidy and nasty...
I want to do the same thing on my Cayman, those axles don't have the vent hole, and they also use a different outer clamp style for the CV Boots and the grease typically oozes out under the outer edge of the cv boot/clamp... I had to add some new grease and a new clamp to the driver side axle some time back, and just this past weekend I had to do it on the passenger side. I want to remove both axles and do the same repacking, but just haven't gotten to it, and also since there is no vent hole, I am not sure how to get all the grease out of that outboard joint. With the vent hole I could just repeatedly dunk the axle in and out of the kerosene, and then the acetone to clean it. I guess I could shoot a lot of brakekleen in there after dunking in the kerosene
