5W-30 vs 10W-40 vs 15W-40: Which Oil Should You Buy?

Quick answer

The numbers on engine oil describe viscosity, or how the oil flows when cold and hot. 5W-30 is usually thinner at operating temperature than 10W-40 or 15W-40. The correct oil is the grade your vehicle manufacturer approves, not simply the thickest or cheapest option.

What the numbers mean

The first number with W describes cold-start flow. The second number describes protection at engine operating temperature. A 5W-30 oil flows well at startup and stays at a 30-grade viscosity when hot. A 15W-40 oil is thicker when hot and is often used in older or hard-working engines where approved.

When 5W-30 may fit

5W-30 is common for many newer petrol vehicles and some diesel applications. It can help with cold-start flow and modern engine tolerances when the vehicle manual allows it.

When 10W-40 may fit

10W-40 is a common general-service grade for mixed driving. It can suit many vehicles where the manual approves it and where a slightly heavier hot viscosity is wanted.

When 15W-40 may fit

15W-40 is often considered for older engines, workshops, or work vehicles where that grade is specified. It should still match the required engine specification.

How to decide

Check the manual, confirm petrol or diesel suitability, compare pack size and price, and ask for help if you are buying for a fleet or workshop. Buying the wrong grade can cost more than buying the right oil the first time.

Compare listed oil products and use the cart-to-quote path for repeat servicing orders.