Join the Car Talk Community!

Today: Oil Numbers Explained

RSS
oil
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. 
Tags (Browse All)
oil

You must be logged in to leave a comment. Login / Signup
Support for Car Talk is provided by:

Donate Your Car,
Support Your NPR Station

...and get a tax break!

Get Started

Find a Mechanic


Go



Submit


Rocket Fuel