A lubricating oil composition, more specifically a lubricating oil composition for heavy duty diesel (HDD) engines having a sulfated ash content of no greater than 1.0 mass %, such as from about 0.7 to 1.0 mass %, a sulfur content of no greater than 0.4 mass %, and a phosphorus content of no greater than 0.12 mass % (1200 ppm), such as from about 0.08 to 0.12 mass %; and a TBN of from about 7 to about 15, which lubricating oil composition includes a major amount of oil of lubricating viscosity, at least about 0.5 mass % of an ashless antioxidant selected from sulfur-free phenolic antioxidants, aminic antioxidants, and mixtures thereof, and a minor amount of overbased metal detergent, wherein at least about 60% of the TBN contributed to the lubricating oil composition by overbased detergent is contributed by overbased magnesium detergent