Today: Oil Numbers Explained

Dear Car Talk

Dear Car Talk | Jan 16, 2014

Dear Tom and Ray:

You were answering a question a few weeks ago about what type of oil to use (5W-30, 10W-40, etc.). My question is: What does the "W" mean? For years, I understood it to mean "weight." But then I read some literature published by Shell Oil Co. stating that the "W" meant "winter," and that "weight" was a misnomer. So, what is your take on this? Thanks!

-- Fred

RAY: It's "winter" Fred. Oil's viscosity -- or thickness -- is described as an oil's "weight," so that's probably why there's confusion about what the "W" means.

TOM: According to the American Petroleum Institute, which is sort of the Vatican of oil, when you see a designation that reads, for instance, 5W-20, it means that the oil acts like 20-weight oil in the summer -- or, generally, in hot weather. And it acts like a lighter, 5-weight oil in the winter, or in cold weather.

RAY: Now, since you're an inquisitive fellow, Fred, you probably want to know why it's not labeled "5W-20S," then?

TOM: Because the "summer" designation is unnecessary. Obviously, if one number is the cold-weather number, the other must be for hot weather. It's why boxes sometimes say "This side up" but don't bother also saying "This side down."

RAY: Wouldn't it be more useful if boxes DID say "This side down" or "Other side up," instead of "This side up"? I mean, once you can see the words "This side up," you already have that side up! What you really need is "Other side up" to tell you when the genuine 4th-century Ming vase you bought on eBay for $49 is upside down and already broken.

TOM: Have we answered Fred's question?

RAY: I think so. I mean, Fred, I'm sure you understand why we have multi-viscosity oil. In the old days, people had to drain out their summer-weight oil in the fall and put in winter-weight oil. That was a pain in the butt.

TOM: But when it's winter, you want a lighter oil so the engine's parts can move through it more easily. In the winter, everything's harder to move.

RAY: Like me, for instance.

TOM: So now, oils miraculously (I think) vary their viscosity to adapt to conditions, so we can spend our time more productively, like surfing eBay for more junk.

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