SAE engine oil viscosity grades
Engine oil grades like 5W-30 come from SAE standard J300. The two numbers describe how the oil flows when cold and when hot — getting the right grade keeps an engine protected from a freezing start-up to full operating heat. This reference explains the format and lets you look up the temperature suitability of any common grade.
How it works
A multigrade oil meets two viscosity targets. The number before the W (winter) sets the cold-flow limit: lower is thinner when cold, so 0W flows at colder temperatures than 5W or 10W. The number after the dash is the operating viscosity at ~100°C, validated by a high-temperature high-shear test at 150°C; higher means a thicker, more load-bearing film when hot. The lookup maps each grade to an approximate lowest safe cranking temperature and its hot-film behaviour, so you can see whether a grade suits your climate.
Tips and examples
In cold climates favour a low W number — 0W-20 or 5W-30 cranks and pumps far better at −30°C than 10W-40. The second number should match engine design: economy cars often specify 0W-20 or 5W-30 for low friction, while high-performance, towing or older engines may call for 5W-40 or 10W-40 for film strength under heat. Never go thicker than the manual specifies; oil galleries and the pump are designed for one grade. Synthetic oils achieve wide grades (e.g. 0W-40) that mineral oils cannot.