Your climate matters less than the number on the oil cap

Quick answer: Use the grade your owner's manual lists. If it gives you a choice, go thinner for cold winters and the upper end of the range for brutal heat, towing, or an engine with a lot of miles. Don't jump outside the manual because a guy at the parts counter swears by it.

Pop the hood and look at the oil filler cap. On most modern cars the grade is printed right on it: 0W-20, 5W-30, something like that. The engineers who built your engine put it there so you wouldn't have to guess.

Most people guess anyway. They read that heat thins oil and cold thickens it, decide their climate demands something special, and walk out of the store with a jug that fights their engine. Here's how the numbers actually work, and where your zip code really does matter.

What the two numbers mean

Oil gets thicker as it gets cold and thinner as it gets hot. A multigrade oil is built to stay in a workable range across that whole swing. Each of the two numbers describes one end of it.

The first number, with the W. The W stands for winter, not weight. It tells you how the oil behaves cold. The lower the number, the better it flows on a frozen morning. A 0W oil is rated to keep pumping at around minus 40, which is the point where Fahrenheit and Celsius agree with each other. A 10W oil gets sluggish well before that.

The second number. This one is thickness at normal operating temperature, measured at 212°F. A 30 is thicker than a 20 once the engine is hot.

So 5W-30 means it flows like a 5W when cold and holds up like a 30 when hot. It's one oil doing two jobs, not a thick oil and a thin one mixed together.

Why the cold number matters most

The first few seconds after a cold start are the hardest on an engine. The oil has drained down to the pan. It has to climb through the pump and the filter and into the tightest passages before anything is properly protected. Thick, cold oil takes longer to get there.

That's why the winter number is the one your climate really touches. If you live where January means single digits, a 0W or 5W oil gets to the top of the engine faster than a 10W will. If you live in Phoenix, the cold number barely matters. You'll never see the temperature where it makes a difference.

And a full synthetic flows better cold than a conventional oil of the same grade. In a hard winter, that's the one place the extra money plainly earns its keep.

The mistake that costs people money

Somebody with a newer car hears that thicker oil "protects better," and puts 10W-40 into an engine that calls for 0W-20.

Modern engines are built with very tight clearances. Many use oil pressure to run things like variable valve timing. They were designed around a thin oil, and part of the reason is fuel economy that the manufacturer has to hit. Run something much thicker and you can slow the flow into those tight spaces, lose some mileage, and add strain on a cold start. If the engine is still under warranty, you've also handed the dealer a reason to argue with you.

Thicker isn't safer. It's different, and the engine wasn't built for different.

Where your climate actually enters

Flip to the oil section of your manual. Many of them print a temperature chart. It might say 5W-30 is fine down to a certain temperature, and 0W-20 is fine anywhere. That chart is your climate answer, written by the people who built the engine.

Some practical guidelines:

• Cold winters and the manual allows a choice: take the lower W number. A 0W-30 over a 5W-30 is an easy call.
• Hot summers, heavy towing, or a lot of stop-and-go in the heat: if the manual lists a thicker hot grade as acceptable, that's where it belongs. If it only lists one grade, use that one.
• A mild climate: stop worrying. Use the grade on the cap.

Check the specs next to the grade, too. Look on the jug for the API "donut" and the current service rating. Some cars need a manufacturer approval on top of it, like GM's Dexos or a European approval number. Those are listed in the manual, and a Euro car that wants an approved oil really does want it.

When the rules bend a little

There are honest exceptions. A high-mileage engine that burns a little oil or has started to seep might do better one step up on the hot number, assuming the manual allows it. High-mileage formulas also carry seal conditioners that some older engines like. If you're thinking of going that way, ask a mechanic you trust who has actually looked at your engine, not the one who's selling you a flush.

Older cars are a different story. A truck from the 1990s was designed for 10W-30 or 10W-40, and putting 0W-20 in it because that's what the new car takes doesn't make sense either. Follow the book for the engine you have, not the one in the next driveway.

Racing, hard towing in the desert, or running an engine that's been modified all change the math. If that's you, you already know to ask someone who builds engines.

Topping off, and a small bit of manners

If you're a quart low on a long trip, add the right grade. If you can't find the exact one, a quart of something close won't hurt you. Mixing grades in a pinch is fine and far better than driving low on oil. Get the correct stuff in at the next change.

And if you borrow somebody's truck and the oil is low, top it off before you hand the keys back. Use the same grade, and tell them you did it. That's the old rule: return it better than you got it. Same goes for the next oil change, which you shouldn't stretch past what the manual says just because the dash light hasn't come on. If you're not sure which maintenance can wait and which can't, this breakdown of the servicing that can't wait will sort it out. And if you're aiming for a car that outlives its payments, what it really takes to get past 250,000 miles puts oil in its proper place in the list.

The ten-minute version

1. Read the oil filler cap.
2. Check the owner's manual for the grade, the approval spec, and any temperature chart.
3. In a hard winter, take the lowest W number the manual allows.
4. Buy a name-brand oil that meets the spec on the label. You don't need the priciest jug on the shelf, but you do need the right label.
5. Write the grade and the capacity on a piece of tape and stick it inside the garage door.

The oil in your engine does a hard job in a hot, violent place. The one decision you really get to make is whether to listen to the people who designed it. Listen to them.