10W-30 is an API/SAE designation for a particular grade of multi-grade motor oil. The 10W indicates that the oil can be used in temperatures as cold as a 10 grade single weight oil could. The 30 indicates that the oil is equivalent to standard 30 grade oil at high temperature (100° C).

10W-30 is the closest thing to a One-Size-Fits-All oil in the auto industry. Many new cars are built to use 5W20, or even 0W20 in some models. As the clearance between part decreases, the oil used must become thinner so an oil film can still form between the parts.

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shaogo says: Say, I just read motor oil and there's no really clear description of what chemicals/reaction *makes* a multi-viscosity oil *change* viscosities according to temperature. And the deal with the stuff getting thicker as it gets hotter has always mystified me.

Well, in response: There's already a wonderful writeup in multigrade oil about how they make this magical stuff (Blackstone Laboratories call them Viscosity Improving Additives), so I changed the hard link above from motor oil to there. A short answer to your second question: Multigrade oil doesn't actually get thicker as it gets hotter, it just doesn't get thin as quickly as single weight oil. 10W-30 works like 10 weight when cold, and 30 weight when hot.

An engine maker that recommends 10W-30 is saying The engine should have SAE30 when at operating temperature, but when the engine is cold (especially in cold climates) SAE30 is too thick; SAE10W is okay for when the engine is cold, but would become too thin at operating temperature. So you need an oil that is both.