Even today there are vast differences in different types of motor oil, and not all are mutually compatible. In the 1920s the oils were more compatible, but stan

Even today there are vast differences in different types of motor oil, and not all are mutually compatible. In the 1920s the oils were more compatible, but standardized specifications had yet to become widespread and home-distilled oils could still be bought in general stores. The amount of gunk between non-detergent and detergent oils, the heat tolerance, the accuracy of the stated viscosity, and even the smell of different oils varied considerably. If you want to remain cynical, you could say that today's consumer propaganda is a continuation of the no-longer-necessary trend that began during a time of fewer clear specifications, regulations, and large monopolies leading to uniform products across an industry and geographical areas.