Service Rating (Classification)
The service rating indicates the quality of your oil; it is independent of the oil's viscosity. Service rating (classification) is defined with two letters according to the API (American Petroleum Institute). The first letter represents a major category. It is either a "S" for Service, and suitable for gasoline engines, or a "C" for Commercial, and suitable for diesel engines. An easy way to remember this is to associate "S" with spark ignition (or gasoline) engines and "C" with compression ignition (or diesel) engines.
The second letter represents the actual rating. Since ratings are assigned alphabetically in order of development, a higher second letter signifies a more advanced oil formulation. This effectively means the oil contains better additives to extend engine life by controlling deposits, sludge, corrosion, varnish, rust and oil thickening. Diesel engine oil ratings may also have a number. Service ratings are as follows with (O) signifying obsolete:
OIL SERVICE RATINGS FOR GASOLINE ENGINES
OIL SERVICE RATINGS FOR DIESEL ENGINESYou should never use oil with an API service rating lower (prior) than that recommended by your owner's manual; this action may void the car warranty. For example, do not use API Service SG oil if API Service SH oil is recommended. However, it is possible and preferable to use oil with higher (subsequent) service rating than that recommended. In Service ("S") oils, subsequent ratings always exceed the required performance properties of previous ratings. For example, an API Service SH or SJ oil could be substituted for an API Service SG oil. In Commercial ("C") oils, subsequent ratings replace previous ratings in specific patterns; see chart above for potential diesel engine oil substitutes.
Unless your vehicle is relatively new, the API service rating recommended in the owner's manual will be superseded or obsolete. This is common because more advanced oils are always being developed. Regardless of what the owner's manual recommends, it is always best to use the highest (latest) service rating available: SJ for gasoline engines and CF, CF-2, or CG-4 for diesel engines.
If the oil is further designated as "Energy Conserving" or "Energy Conserving II," it has advanced additives that reduce friction further and therefore increase mileage by at least 1.5% or 2.7%, respectively. However, your actual results may vary due to engine condition, oil condition, engine manufacturing variances, maintenance habits, operating conditions, and driving habits.
The label may also state "Turbo Approved." This means that it can withstand the tremendous heat generated by a turbocharger. The oil has extra anti-oxidation additives and will not break down when lubricating the turbocharger bearings.
SYSTEMS
ENGINE OIL
MULTIGRADE OILS
FIGHTING OIL CONTAMINATION