That's a sign that the CVT is going, the bearings burn up on them and it turns the fluid dark from the heat in the gearbox. 2006 was the cut-over year and a lot of these CVT's go south.
Consumer complaints about Mini Cooper
$5500 for the transmission at cost from BMW, no rebuild parts available on them at all. I've seen a few $20k paperweights at this point, sadly. I hope yours hangs in there