Dear Car Talk:

Can I use any grade of gasoline for my 2015 Lexus ES 350? A friend just told me I was wasting my money by filling the tank with mid-grade and an occasional tankful of premium. Won't using regular gas harm the car? -- Dee

No. Regular is exactly what your car needs, Dee.

People mistakenly think of gasoline grades as "good, better and best." That's because gasoline companies have done a masterful job of marketing. By calling the higher-octane fuels "Premium," they've convinced people that higher-octane is somehow better. It's not.

Instead, think of octane like your shoe size. If you wear a size 9, would you buy a size 13 because it has a higher number and therefore has to be better? Of course not. You'd end up with blisters. And maybe some extra money from working as a birthday party clown.

The octane rating is a measure of the temperature at which the fuel will detonate inside the cylinder. Every engine is designed to be used with fuel of a specific octane.

You don't want fuel with a lower octane than is required (because you can get pinging, or pre-ignition, which can damage the engine), but you don't want fuel with a higher octane, either (because you'll be throwing away your money).

And since Lexus says, in the owner's manual, that your 2015 ES 350 is designed to run on 87-octane fuel, that's what you should get.

In the old days, you used to hear that using higher-octane fuel had all kinds of beneficial properties: It would clean out your carburetor; it would remove carbon from your pistons; it contained special detergents to clean your fuel injectors; it would regrow hair.

All of those are false. Cars don't have carburetors anymore. Carbon doesn't build up on pistons. And fuels of all grades contain detergents.

So save your money, Dee. Buy the grade of fuel recommended by your manufacturer, and wear the right size shoes, too.