The VC is a sealed unit that is immersed in the same oil as the differential.
The differential oil conducts heat away from the VC unit. Synthetic gear oils
are said to be better conductors of heat than conventional gear oils (I think
I read that on the RedLine web site). If that is true, then a synthetic gear
oil would help the VC to cool more efficiently and may help it live longer
(than its apparent 100K threshold).
-Larry