Start with gasoline or diesel
API S categories are mainly for gasoline engines; C and certain F categories are for diesel engines. An S marking alone does not establish suitability for diesel engines.
Newer does not mean interchangeable
Newer gasoline S categories generally address newer test requirements, but suitability still depends on the owner's manual, viscosity and manufacturer approvals. Diesel categories such as FA-4 and CK-4 are not freely interchangeable.
How do you compare them?
If the manual specifies SAE 5W-40 and API SN, the product must meet both requirements. A 5W-40 marking alone, or API SN alone, is not enough.
Keep in mind
Write down the complete viscosity, API/ACEA/JASO requirements and manufacturer approvals, then compare each one with the label.
Reference:API Motor Oil Guide