American Petroleum Institute (API) Engine Oil Specifications
API (American Petroleum Institute) specifications are often seen on oil containers and they show the minimum standard of oil that can be used in an engine.
You can't really use the API specifications to measure the quality of an oil as many of the top end ester based oils meet API SL rather than the latest API SM specification.
S = Service - Petrol Engine Performance
C = Commercial - Diesel Engine PerformanceA

Petrol Engine Specifications
SG - Introduced 1989 has much more active dispersant to combat black sludge.
SH - Introduced 1993 has same engine tests as SG, but includes phosphorus limit 0.12%, together with control of foam, volatility and shear stability.
SJ - Introduced 1996 has the same engine tests as SG/SH, but phosphorus limit 0.10% together with variation on volatility limits
SL - Introduced 2001, all new engine tests reflective of modern engine designs meeting current emissions standards
SM - Introduced November 2004, improved oxidation resistance, deposit protection and wear protection, also better low temperature performance over the life of the oil compared to previous categories.
SN - Introduced in October 2010 for 2011 and older vehicles, designed to provide improved high temperature deposit protection for pistons, more stringent sludge control, and seal compatibility, performance with improved fuel economy, turbocharger protection, emission control systemcompatibility, and protection of engines operating on ethanol-containing fuels up to E85.

Diesel Engine Specifications
CD - Introduced 1955, international standard for turbo diesel engine oils for many years, uses single cylinder test engine only
CE - Introduced 1984, improved control of oil consumption, oil thickening, piston deposits an wear,uses additional multi cylinder test enginesAPI Specifications
CF4 - Introduced 1990, further improvements in control of oil consumption and piston deposits, uses low emmission test engine
CF - Introduced 1994, modernised version of CD, reverts to single cylinder low emission test engine.Intended for certain indirect injection engines
CF2 - Introduced 1994, defines effective control of cylinder deposits and ring face scuffing, intended for 2 stroke diesel engines
CG4 - Introduced 1994, development of CF4 giving improved control of piston deposits, wear,oxidation stability and soot entrainment. Uses low sulphur diesel fuel in engine tests
CH4 - Introduced 1998, development of CG4, giving further improvements in control of soot related wear and piston deposits, uses more comprehensive engine test program to include low and high sulphur fuelsSG - Introduced 1989 has much more active dispersant to combat black sludge.
CI4 - Introduced 2002, developed to meet 2004 emission standards, may be used where EGR ( exhaust gas recirculation ) systems are fitted and with fuel containing up to 0.5 % sulphur. May be used where API CD, CE, CF4, CG4 and CH4 oils are specified.
CJ4 - Introduced in 2006 for high-speed four-stroke engines. Designed to meet 2007 on-highway exhaust emission standards. CJ-4 oils are compounded for use in all applications with diesel fuels ranging in sulphur content up to 500ppm (0.05% by weight). However, use of these oils with greater than 15ppm sulfur fuel may impact exhaust after treatment system durability and/or oil drain intervals. CJ-4 oils are effective at sustaining emission control system durability where particulate filters and other advanced after treatment systems are used.

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The team at Opie Oils