Bruce, thanks for the reply.
The fact that RL is such a shear stable oil is one of the reasons I use it in my Caterham for track and fast road work in the summer with total confidence. But that isn't my concern during the dead of winter up hear in the Great White North.
For my daily driver so much of the time the oil is not up to temperature so even the lightest oil I can get my hands on is technically too thick most of the time; i.e. M1 0W20 which is much lighter than RL 5W20; their lightest oil.
I've managed to get the viscosity calculator to work and have determined the kinematic viscosity of RL's 5W30 and 10W30 at -20C to be 3954 cSt and 4404 cSt respectively. In comparison M1 0W30, 5W30 & 10W30 vis' are 1995 cSt, 2225 cSt & 3425 cSt; significantly lower to say the least.
When it comes to sub zero performance this does not appear to be what RL is best at.
Peter