What is the composition of API Group III base stocks often marketed as synthetic motor oils?

Highly refined mineral oil (Hydrocracked/Severely Hydroprocessed)

API Group III base stocks represent a significant category in the modern synthetic landscape, though their origins differ fundamentally from the original synthetic materials. These oils start as conventional mineral oil but undergo extremely rigorous refining processes, specifically hydrocracking or severe hydroprocessing. This intensive treatment chemically restructures the hydrocarbon molecules to achieve properties that make them chemically similar to true synthetic PAOs (Group IV) regarding thermal stability and consistency. However, because their foundation remains crude oil derived, they are classified differently than the true synthetic Group IV PAOs, although they offer superb protection for most modern vehicle requirements.