My understanding is that the '20' is the viscosity of the base oil and the '50' is the virtual viscosity arrived at by the addition of additives and processing. Therefore the '50' is the viscosity that is effective with the engine hot and the '20' is the cold-start viscosity.

Now you've got me all exercised - I'm going to have to look it up!

__________________
1991 300 SEL
218,000 Km

"Xsbank's rules of mechanical intervention: Always go for the easiest solution first; 90% of what ails a gasoline engine is ignition; After that its all a WAG."