How often should I change my engine oil?

This depends on two factors:

  • Driving conditions – long drives, short runs etc.
  • API service level.

Longer drives will result in higher mileage from engine oils whilst shorter drives reduce the service interval. Higher API service ratings ensure longer oil service intervals.

API service levels and SAE viscosity grades

SAE grades were created as numeric codes to classify different viscosity oils. For example, a monograde SAE 40 oil means that this oil viscosity at 100°C should be between 12.5 cSt and 16.3 cSt. Multigrade oils have the alphabet “W” between two numbers. This denotes Winter Grade and performs better than monograde oils upon cold start up as it is “thinner” at cold temperatures. The important thing to remember is that as the SAE numbers become larger, the viscosity increases. API certification requires that oils meet certain OEM quality and performance standards. There are API specifications for both Petrol (“S” category) and Diesel (“C” category) engines. Petrol specifications first started with SA. “S” signifying petrol, and “A” meaning that this was the first performance standard ever invented. Then came SB, SC, SD and so on. So as we go further along the alphabets, the oil spec is higher. The current leading spec for petrol is SN. This pattern is the same for diesel specifications. The lowest spec. turbo diesel oil will have an API rating of CF-4, then comes CG-4, CH-4 and CI-4. Let’s look at our 15W40 example: