Which mineral oil has the highest quality base stock?

Group III mineral oils are recognized for having the highest quality base stock among the categories of mineral oils. These oils undergo extensive refining processes, which significantly reduce impurities and enhance their performance attributes. Unlike Group I and Group II oils, which may still contain certain undesirable elements and have lower oxidative stability, Group III oils offer better thermal and oxidative stability due to their superior refining techniques.

Moreover, Group III oils generally have a higher viscosity index and better low-temperature properties compared to lower-group oils, making them more effective under varying temperature conditions. This allows for better lubrication and protection of machinery components over a broader range of operating environments.

Group IV oils, often categorized alongside synthetic oils (e.g., PAOs or polyalphaolefins), do exhibit high-quality properties as well but are fundamentally different in composition, often not classified solely as mineral oils. This distinction is essential in understanding the context of the question, which focuses specifically on mineral oils and their group classifications.