Motor oil grades look like a secret code, but they describe something simple: how easily the oil flows when the engine is cold and how well it holds up when the engine is hot. Get the grade right and your engine stays protected in every season.
What the two numbers mean
Take 5W-30 as an example. The 5W is the cold rating, where W stands for winter. A lower first number flows better on a freezing morning, which is when most engine wear happens. The 30 is the hot viscosity, or how thick the oil stays at full operating temperature. A higher second number gives a thicker film at heat.
5W-30 vs 10W-40
5W-30 flows more easily on cold starts than 10W-40, so it reaches the top of the engine faster in winter and suits most modern cars. 10W-40 is thicker cold and hot, which some older or higher mileage engines prefer because the heavier film can quiet things down and hold pressure. Always start with what the owner manual specifies.
Should you use synthetic?
Full synthetic oil flows better cold, resists breakdown at heat, and lasts longer between changes than conventional oil. For most drivers it is worth the extra cost, especially in extreme temperatures or with a turbocharged engine. You can compare options in our engine oil guides.
Frequently asked questions
Can I switch from 10W-40 to 5W-30?
Only if your manual lists 5W-30 as an approved grade. Do not go thinner than the manufacturer allows just to save fuel.
Is thicker oil always better for an old engine?
Not always. A slightly thicker grade can help a worn engine that burns oil, but too thick starves cold starts. Move up one step at most.
Does synthetic cause leaks in old cars?
Modern synthetics are compatible with normal seals. Very old engines with hardened seals may show a weep, but the oil is not damaging anything.
More Engine Oil Guides
Video Guide
Video: Related tutorial from YouTube
