These letters, such as API SJ and so forth denote the oil's service grade. It does NOT denote the quality of the oil.

API standards for American Petroleum Institute. They work together with automobile manufacturers to determine the best formulation for engine oils. For example, the latest API SN standard caters to vehicles with Euro 5 emissions rating. Engines that conform to this standard operate at higher temperatures to burn off fuel more completely, hence the need for oils to withstand this temperature.

By the way, the API S-series of grades are for gasoline engines.

Other criteria include (among other things):

• Resistance to viscosity change due to heat.
• Compatibility with gaskets.
• Resistance to the use of "engine treatments."
• Resistance to the oil growing thicker.
• Resistance to the formation of sludge.

As mentioned above the latest grade is API SN which was introduced. However, previous grades such as API SJ, SL, SM can still be used as there are engines with Euro 4 and below on Malaysian roads.

Grades SA through SH were formulated many decades back and are now obsolete and no longer in use.