Which series of engine oils are used in diesel engines?

• A. A Series A
• B. B Series B
• C. C Series C
• D. D Series D

Answer: (C)

Diesel engines operate under harsher conditions, with more soot, higher temperatures, and longer drain intervals, so they need oils formulated to handle those stresses. The C Series is designed for diesel service, with an additive package that helps keep soot from thinning the oil, protects bearings and pistons under heavy loads, and maintains viscosity and cleanliness at high temperatures. That combination of detergents, anti-wear properties, and oxidation resistance makes C Series oils the appropriate choice for diesel engines. The other series are geared toward different engine types or uses, and wouldn’t provide the same protection in diesel conditions.