I believe it is a specific oil. I always thought the long average lifespan on the mercedes cv joints was attributed to using oil. Grease tends to displace after awhile, goes hard sometimes tool.

Grease does not like to flow at all at really cold temperatures. Oil probably does not either but it would respond faster to any warmth generated in the joint.

Again, whats known is the joint is packed with CV grease, the question is is a small amount of gear oil/other added to each boot when replacing/repacking joints/boots?