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 to read the grade format
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.
How the two numbers work together
Think of a multigrade oil as behaving like a thin oil in the cold and a thicker oil once warm. This is achieved through the base oil formulation and viscosity index improvers — polymer additives that resist thinning as temperature rises. SAE J300 defines the exact cold cranking viscosity limit (in mPa·s at specific subzero temperatures) and the kinematic viscosity range at 100°C for each grade number.
For example, a 5W-30 oil must pump adequately at −35°C (the 5W cold limit) and maintain a kinematic viscosity of 9.3 to 12.5 mm²/s at 100°C (the 30 band). Synthetic base oils achieve this across a wider temperature range than mineral oils, which is why 0W-20 and 0W-40 grades are only possible with synthetic formulations.
Matching the grade to your engine and climate
Cold climate priority: choose a low W number. A 0W or 5W grade protects the engine in the first seconds after a cold start, when the oil pump is moving thick oil through narrow galleries before it warms up. Engine wear is disproportionately concentrated in those early seconds.
Hot-running load priority: the second number matters for towing, sustained high RPM, or older engines with wider tolerances. A 40-grade holds a thicker film under heat and shear than a 20-grade, which is why high-output and diesel engines typically require 5W-40 or 10W-40.
Fuel economy: thinner hot grades (0W-20, 5W-20) reduce pumping losses and friction, which is why modern passenger cars increasingly specify them. The trade-off is reduced film thickness under extreme load.
Never go thicker than the manual specifies. Modern engines with variable valve timing, turbochargers, and tight oil galleries are calibrated for one grade; using a heavier oil can starve components during cold starts and increase fuel consumption. Using the specified synthetic grade, even if it costs more, is always the correct choice over a cheaper mineral oil in a different grade.