Automotive Reference
"5W-30" is two viscosities in one bottle: how it flows cold, and how thick it stays hot. Here's what each number means and which grade goes where.
Multigrade oil is rated at two temperatures. The first number (with a W) is the cold rating; the second is the hot rating at full engine temperature.
Lower "W" = better cold flow. Higher second number = thicker when hot. Always use the grade printed on your oil cap or in the manual — the engine was designed around it.
| Grade | Cold-flow | Typical use |
|---|---|---|
| 0W-20 | Excellent (very cold) | Modern fuel-economy engines |
| 5W-20 | Very good | Many current gas engines |
| 5W-30 | Very good | The most common all-rounder |
| 10W-30 | Good | Older engines, warm climates |
| 10W-40 | Good | Older / higher-mileage engines |
| 15W-40 | Mild winters only | Diesels, heavy-duty |
| 5W-40 | Very good | European / performance, diesels |
It's a multigrade oil. The 5W is its cold (winter) rating — how easily it flows and pumps on a cold start; lower is better in the cold. The 30 is its viscosity at 100°C, full engine temperature — how thick it stays when hot. So 5W-30 flows like a thin 5-weight cold and protects like a 30-weight hot.
Winter. It marks the cold-temperature rating, not "weight." 0W and 5W flow well in deep cold; 15W and 20W are stiffer when cold and suited to warmer climates.
They flow the same cold, but 5W-30 is thicker when hot. In a pinch it won't hurt short-term, but the engine was tuned for 5W-20 (fuel economy, oil-passage sizing), so it's not a free swap — and it can affect warranty. Go back to the specified grade at the next change.
Yes — at operating temperature. A 40 is thicker hot than a 30. Thicker isn't automatically better; too thick for the engine's clearances reduces flow and can raise wear and temps.