Not every oil marketed as “full synthetic” is made from the same type of base stock. Many synthetic oils sold in the United States use highly refined Group III petroleum base oils, while true Group IV PAO synthetics are chemically built for improved thermal stability, oxidation resistance, cold-flow performance and lower volatility. The difference became blurred after the late-1990s Mobil–Castrol advertising dispute allowed Group III oils to be marketed as synthetic in the U.S. European specifications are also based mainly on finished-oil performance, and terms like “Euro blend” do not automatically mean PAO-based. The best oil choice depends on the vehicle, required specifications and how it is actually used.