Engine oil explained: what 5W-30 and 15W-40 really mean

Oil is the cheapest protection your engine gets, and the wrong oil or a late change is behind many expensive repairs.

Reading the label

• The first number with a W (0W, 5W, 15W) is how easily the oil flows when the engine is cold. Lower flows faster at start-up, when most wear happens.
• The second number (20, 30, 40) is how thick it stays at full operating temperature.
• The specification (for example API SP for petrol, or a diesel specification such as API CK-4) matters as much as the grade. Your manual lists what your engine needs.

Which one?

Use the grade and specification in your owner's manual. Modern Japanese petrol engines are often designed for 0W-20 or 5W-30; older engines and many diesels use thicker oils such as 15W-40. A thicker oil isn't "more protection" for an engine designed for a thin one.

How often?

Most manuals give a "normal" and a "severe" interval. Kenyan conditions (dust, heat, stop-start traffic, short trips, heavy loads) are usually severe, so 5,000 km is a sensible default unless your manual and mechanic say otherwise. Change it at least once a year even if you drive little.

Tips

• Check the dipstick every couple of weeks with the engine off, on level ground.
• Buy from reputable outlets; counterfeit oil is a real problem.
• Always replace the oil filter at the same time.