What the base oil groups are
Every finished motor oil is built from a base oil plus an additive package. The American Petroleum Institute (API) classifies base oils into five groups based on how they are processed and their chemical properties such as saturate content, sulphur content, and viscosity index.
Groups I and II are mineral oils refined directly from crude, with Group II being more refined and more stable than Group I. Group III is a mineral oil that has been very heavily refined (hydrocracked), giving it properties close to a synthetic. Group IV is polyalphaolefin (PAO), a chemically synthesized base oil. Group V is a catch-all for base oils that do not fit the other groups, such as ester-based stocks, and these are often used in blends rather than alone.
Why ‘synthetic’ on the label varies
The word ‘synthetic’ is not tightly defined the same way everywhere. In many markets a Group III base oil can legally be sold as ‘synthetic’ or ‘full synthetic’ because its performance approaches that of a PAO, even though it began as crude oil. A Group IV PAO oil is synthetic in the strict chemical sense. This is why two bottles both labeled ‘synthetic’ can be built on different base stocks.
For most drivers this distinction matters less than it sounds. Both well-made Group III and Group IV oils can deliver excellent protection, and either can meet demanding specifications.
How this relates to choosing oil
The base oil group describes quality and chemistry, but it is separate from viscosity grade (the SAE number) and from the specifications the finished oil meets (API, ILSAC, ACEA, or an OEM approval). When choosing oil, do not chase a particular group. Instead, match the grade and the specification listed in your owner’s manual, and let the manufacturer’s blend handle the base stock.