After making some calls here is what I was told by a reliable suspension guy. 01 is made for open cartridge forks. When pored out it would look like a 10wt. It was designed to operate under cavitation and foaming. It can be used in the newer forks but would be really stiff. S1 is a 5wt and designed to work in the twin chamber system. There was a little bit more to it, and something about the air in the old forks.