Possibly the initial motor oil performance enhancers were actually Motor Oil viscosity index modifiers: Remember STP...? In those times they were simply identified as motor oil additives.

What's a motor oil Viscosity index modifier?

(VI) is a popular technique of calibrating a fluid's change of viscosity with regards to temperature. The greater the VI, the lesser the relative change in viscosity with temperature. VI improvers (also called viscosity modifiers) are additives that improve the viscosity of the fluid all through its useful temperature range.

Viscosity modifiers are polymeric molecules that are sensitive to temperature. At low temperatures, the molecule chain shrinks and does not effect the fluid viscosity. At higher temperatures, the chain unwinds