Originating in the 1930s, the least refined type which is produced by Solvent Refining. It usually consists of conventional petroleum base oils. An improvement to the refining process in the 1960s called hydro-treating made this base oil more stable, less reactive, and longer lasting than the earlier base oils. API defines group I as “base stocks contain less than 90 percent saturates and/or greater than 0.03 percent sulfur and have a viscosity index greater than or equal to 80 and less than 120”.
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