Different grades of Motor Oil... We are used to Multi-weight Oils such as 5-30, 10-40 etc...
Back in the 20s through late 30s, there were no multi-weight Oils... The motorist would change to a lower number oil in winter and a higher numbered oil in the summer.... Also some manufacturers used a "W" behind oils marked 5W,10W,15W etc...
As I understand it,Mobiloil to be unique, labeled their oil ; Mobiloil A,B, etc.... (or atleast that is the way, I understand it... I maybe spreading some misinformation. If so those with more knowledge than I,will correct me)
Wikipedia, does a better job describing what I am attempting too:
http://en.wikipedia.org/wiki/Motor_oil(I see Darin, beat me too the answer... LOL)