It is possible for the rollers to not roll anytime roller bearing friction is much greater than the cam/roller contact friction. This can definitely happen due to bad contact geometry or failing roller bearings. Seems less likely that it would be an oil lubricity issue but I have never seen any data to really back that up. Friction is based largely upon viscosity so I don't see a concrete reason how friction is that different between synthetic and dino oils with the same viscosity. However if you get into boundary layer friction other factors may apply. Confused, yeah me too.

Kitz