5W-30 and 5W-20 behave identically in winter cold-starts (both are 5W), but 5W-30 is slightly thicker at full operating temperature while 5W-20 is thinner for better fuel economy. Use whichever grade your owner's manual specifies — they are not freely interchangeable.
Attribute
5W-30
5W-20
Winter / cold-start flow Both pump the same when cold.
5W (identical)
5W (identical)
Hot viscosity (100 °C)
30 — slightly thicker film
20 — thinner, lower drag
Fuel economy
Marginally lower
Marginally better
Typically specified for
Many engines calling for a 30-grade
Engines tuned for 5W-20 economy
Bottom line: Same cold rating, different hot thickness. Follow the grade your manual specifies; do not pick by "thicker is safer."
The one real difference
Both oils share the same 5W winter rating, so cold-start protection is effectively the same. The only meaningful difference is hot viscosity: 5W-30 keeps a slightly thicker film at operating temperature, while 5W-20 runs thinner to reduce drag and improve fuel economy.
Which should you use?
Use the grade your owner’s manual specifies for your engine and market. Some manuals list both as acceptable across a temperature range — in that case either is fine within the stated conditions. If only one is listed, use that one.
Frequently asked questions
Is 5W-30 thicker than 5W-20?
Only when hot. At operating temperature 5W-30 is slightly thicker (30 vs 20). When cold, both flow the same because both are 5W.
Can I just use 5W-30 because it's thicker?
Not unless your manual lists it. "Thicker is safer" is a myth — the manufacturer tunes the engine for a specific grade, and the wrong viscosity can hurt fuel economy and, in some cases, protection.