Tech Session

Richard Simonds

The Best Oil for Your Car A Quick Reference

The article on engine oil in the September-October 2012 issue of The Star led to numerous responses and inquiries. In this synopsis are the key characteristics that determine the type of oil you should select for your Mercedes-Benz, remembering that these engines fall into four different technologies by era in terms of engine design, emissions systems, and manufacturing technology.

Engines built before World War II typically had large clearances between piston rings and cylinders and were built with fairly large tolerances between moving parts. Engines built from 1947 to 1975 typically have flat tappets rubbing directly against camshafts for valve operation and larger cylinder clearances; engines built from 1976 to 1997 have catalytic converters and rolling cam followers operating the valves; and engines built 1998 to the present have rolling cam followers, catalytic converters, and precision cylinder clearances.

With 480 companies certified by the American Petroleum Institute (API) to produce engine oil in the United States – and each one producing 75 to 150 different formulations – there are literally thousands of oils from which to choose. Consequently, it would be difficult to recommend specific brands for your use. Instead, this reference guide shows you what to look for in a concise way as you select the best engine oil for your specific Mercedes-Benz vehicle.

The key characteristics of oil are:

• Classification by engine type: S = spark (gasoline) and C = compression (diesel) (which also may be used in older gasoline engines).
• Formulation of oil: SN, SM, SL, SJ, etc. for gasoline and CJ, CJ-4, CI, CI-4, CH, CH-4, etc. for diesel engines: refers to additives used in different periods of time, including ZDDP (the family of zinc and phosphate additives that provide wear protection in engines, but clog catalytic converters). SN, SM, CJ have lower levels of ZDDP; SL, SJ, CI have higher levels of ZDDP; “PLUS” oils reduce soot-related viscosity change in diesel engines and, thus, are not recommended for older gasoline engines. Do not use “heavy-duty truck” oils with high ZDDP in automobile engines because they can clog the smaller oil passages, and automobile engines don’t get as hot as truck engines and, thus, the truck oil does not provide good lubrication when warm.
• Catalytic converter compatibility: In general, oils that have been formulated to be compatible with catalytic converters are marked on the container with an American Petroleum Institute starburst symbol, as below.
• Base stock: Oils are made from a mineral base, or a synthetic base. (Semi-synthetic – blended – base oils are not recommended).
• Viscosity: Oils are rated by a number 0, 5, 10 followed by a W that indicates the oil flow rating at cold start, and in multi-viscosity oils, with a second number – 20, 30, 40, 50 – that is the oil flow rating at (higher) operating temperatures (Shown on oil containers as: 0W, 5W, 10W, 20W, etc., or 0W-20, 5W-20, 10W-30, 10W-40, 20W-50, etc.).

Note that all oils require the engine to be running at operating temperature before maximum protection starts. Thus, heavy acceleration and high speed driving should not be done until the engine has reached operating temperature, which is most efficiently achieved by driving the car under minimal load for 15-20 minutes.

Always choose a viscosity for the climate you are in and your driving patterns. As illustrated in the diagram, cold climates and cold start conditions require a low viscosity at start (10W for engines built before 1998, 0W or 5W for newer engines). Hot climate, hot start, or high-speed driving conditions require higher viscosity ratings. Multi-grade oils give you the good flow at cold start with protection for high-temperature driving. Refer to your owner’s manual for recommendations and in newer cars to the Factory Approved Service Products booklet.

Quick Reference Recommended Types of Oil

The quick reference table below shows the combination of characteristics that is preferred for engines in each category.

API Rating and Starburst Symbols: