DEAR CAR TALK: I change the oil in our cars, mainly because I’m too cheap to pay what oil-change shops charge.

Is there any way to test to see when the oil in a vehicle is almost worn out? Our cars vary widely in their use. One does a daily 15-mile commute; another goes 60,000 highway miles a year; and a van goes 3,000 miles or so a year, at times pulling a small camper.

Sometimes the oil I drain looks like new. With oils and filters being so good these days, I’m wondering if I’m wasting time and money. If there were some litmus test for oil, it would be helpful, rather than simply going by miles. Thanks from a fan for decades. – Pat

RAY: There’s not really a good litmus test, Pat.

For ages, we’ve always estimated with miles and months. For a long time, our recommendation was to change the oil and filter every three months or 3,000 miles.

But that recommendation is completely outdated now. With conventional oil, you can go six months or 7,500 miles. And with synthetic oil, you can go 10,000-12,000 miles, or a year. Some say more.

But now a lot of cars have their own, built-in oil life indicators. They work in different ways, depending on the manufacturer. Some use a direct measurement of some kind, testing the conductivity of the oil, the soot concentration or the presence of water. Other systems keep track of your mileage, the number of times you start the car and the temperature conditions under which you drive.

They feed all of that data into an algorithm, and then tell you when it’s time to hit Pokey Lube.

Those systems seem to work well, and can help you cut down significantly on the frequency of your oil changes, based on real evidence rather than guesswork. So you might want to make sure that your next vehicles have those systems, Pat.

As for an aftermarket “litmus test” wiping some magic test strip on the dipstick to find out how much oil life remains and whether you soon will meet the girl of your dreams – I haven’t found anything I’d be willing to really rely on yet, given that the downside is a ruined engine.

If I were you, I’d switch to synthetic, just to reduce the amount of time you spend lying under those three cars with hot oil running down into your armpit. And keep changing the oil based on your best estimates. After all, even several extra oil changes over the life of a car are cheaper than an engine rebuild.

Got a question about cars? Email Car Talk’s Ray Magliozzi by visiting the Car Talk website, www.cartalk.com.