I found this link. It is a little old, but I found the description of the Viscosity interesting. However, for the cars we are in this forum for it is not at all too old.
An nice synopsis of the labelling history. Sadly lacking citations.
IMO, the best overview of gasoline engine oil properties for those of us not in tribology is Widman's.
And for those involved with or interested in racing engines, A.E. Haas's white paper on viscosity, temperature and volumetric flow is very worthwhile too.
There are links to both on my website or I can copy here if you'ld like.