If you've ever picked up a bottle of engine oil and seen a code like API SN or API CI-4+ stamped on the label, you've seen an industry specification — a shorthand that tells you exactly what performance level that oil is certified to meet. Here's how to actually read one.

API service categories: the letter tells you the fuel type

The American Petroleum Institute (API) rates oils with a two-letter code. The first letter tells you the category:

  • S (Service) — for petrol/gasoline engines, e.g. SJ, SL, SM, SN, SP. Each successive letter is a newer, generally more demanding standard.
  • C (Commercial) — for diesel engines, e.g. CD, CF-4, CH-4, CI-4, CI-4+.

A newer letter (SP is newer than SN, which is newer than SL) generally means the oil meets tighter requirements for deposit control, wear protection, and — in recent categories — fuel economy and emissions-system compatibility. As a rule of thumb, an oil certified to a newer category can normally be used where an older category is specified, but not always the reverse — always check your vehicle or equipment manufacturer's handbook.

GL-4 vs GL-5: gear oil is a different scale entirely

Gear oils use a completely separate API scale — GL-1 through GL-5 — rating extreme-pressure (EP) additive content, not the same wear/deposit criteria as engine oils.

  • GL-4 is formulated for moderate-speed, moderate-load conditions — common in manual transmissions and some transaxles.
  • GL-5 carries a higher concentration of EP/anti-scuff additives, intended for hypoid differentials operating under higher speeds and shock loads.

Because GL-5's additive package is more aggressive, it isn't automatically a safe substitute for GL-4 in every application — some yellow-metal (brass/bronze) synchromesh components are more sensitive to GL-5's sulphur-phosphorus additives. Always match the grade your equipment manufacturer specifies rather than assuming "higher number is always better."

Where to find AGREX's certified grades

Every AGREX engine oil, diesel oil and gear oil page states its exact API, JASO, ILSAC and BIS certification in the Technical Specifications tab. Browse the full range on our Automotive Engine Oils & Transmission Fluids family page, or contact our technical team if you're not sure which grade your equipment needs.