Multigrade
Multigrade engine oil carries a viscosity improver that lets one oil meet a cold-start grade and a hot operating grade, which is why grades are written as 15W-40 or 20W-50 rather than a single number.
The first number with the W is the cold-cranking rating, and a lower figure means the oil flows more readily at start-up, when most engine wear happens. The second number is the viscosity at operating temperature, and it needs to be high enough to hold an oil film across the bearings and cylinder walls under load. In South African conditions 15W-40 remains the mainstay for diesel fleets and older petrol engines, while 20W-50 is still used in high-mileage engines with looser clearances.
Always match the multigrade engine oil to the API or ACEA level in the vehicle handbook, not just the viscosity grade.
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