Oil Viscosity Selector
What 5W-30 actually means, and which grade suits your climate and engine. The manufacturer's spec wins, but this explains why.
Your conditions
- The manufacturer's specification is not a suggestion. Modern engines are designed around a specific viscosity.
- The W number is a cold flow rating, not a separate weight. 5W-30 is one oil, not a blend of two.
- Meeting the right specification, such as an API or ACEA rating, matters as much as the viscosity numbers.
What 5W-30 actually means
It is one oil with two ratings. The 5W describes how it flows when cold, with W standing for winter. The 30 describes its viscosity at operating temperature, around 212F.
A multigrade oil is not thin oil mixed with thick oil. It is a base oil with viscosity modifiers that keep it flowing at low temperature while still behaving like a 30 weight when hot. A lower W number means better cold flow, which is where most engine wear happens.
Cold start is where wear happens
The great majority of engine wear occurs in the first seconds after a cold start, before oil reaches the top of the engine. That is the entire reason the W rating exists, and why 0W oils are worth having in a cold climate even though they cost more.
Do not go heavier on a modern engine
Engines specified for 0W-16 or 0W-20 have narrow oil galleries and variable valve timing actuators that are hydraulically operated by engine oil. Putting heavier oil in slows those actuators and restricts flow where the design assumes it will not be restricted.
The old habit of stepping up a grade for a worn engine or a hot climate came from an era of looser tolerances. It still has a place on older designs. It does not on a modern one that specifies a low viscosity oil.
Frequently asked questions
What does the W in 5W-30 mean?
Winter. It is a rating for how the oil flows at low temperature, tested cold. The second number is viscosity at operating temperature. A 5W-30 and a 10W-30 behave the same when hot, but the 5W flows better on a cold start.
Can I use 10W-30 instead of 5W-30?
In a warm climate, usually with no ill effect, and many manufacturers list both. In cold weather the 10W is measurably worse at getting oil up to the valvetrain quickly on start-up. There is no advantage to the heavier winter rating, so there is little reason to choose it.
Should I use thicker oil in an old engine?
Sometimes. A worn engine with larger bearing clearances can hold pressure better on a heavier grade, and it may slow consumption. It is a way of managing wear, not fixing it, and it does not apply to a modern engine specified for low viscosity oil.
Does thicker oil protect better?
Not automatically. Oil protects by maintaining a film between moving parts and by getting there quickly. Too thick and it does not flow into tight clearances fast enough on start-up, which costs more wear than the extra film thickness saves.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.