Clem, what are you referring to as 600 weight? The stuff that is sold to the Model A guys? If so, several things here, first "w" doesn't stand for oil weight in general, it is for "winter" and has to do with the temperature the viscosity is measured at. Second the "w" in 600w is a brand name for a Mobile steam engine oil and was rated under a different system than what is currently used. It has a viscosity of around 140 under current SAE standards. IMO it is mostly a gimmick and nothing I would use for much of anything.
