Car Talk: Ignoring oil changes wasn't a slick move

Q: I have not had the oil changed in my 2011 Hyundai Tucson since 2015. I put lots of short trips on this car, and it doesn't even have 40,000 miles on it. Am I negligent?

A: We give you credit for coming forward and fessing up, but do you really have to ask if this has been wrong?

The reason we change our oil every 5,000 or 7,500 miles (or 10,000 or more if it's synthetic oil) is to keep the engine properly lubricated. With the metal parts inside the engine rubbing against each other thousands of times per minute, good lubrication is the difference between your engine having a long life and a short life marred by lots of burning oil.

And oil provides more than just lubrication, as crucial as that is. Oil also picks up contaminants and dirt inside the engine and holds them in suspension. If the oil gets saturated with dirt and can't absorb any more, that dirt's going to stay in your engine. And in the worst cases, we've seen more than an inch of sludge in the valve train. Those engines are toast.

You might be lucky. Maybe the automotive gods were smiling on you, and, despite not changing the oil for the past 20,000 miles or so, there's no sludge in your engine. Then all you have to worry about is that poor lubrication will lead to oil burning down the road. And you can solve that problem by selling the car to your brother next week. That's what I always did.

But if it were me, I'd want to know. I'd ask my mechanic to take off the valve cover and peek in there. If there's an inch of sludge in there, you're either looking at an engine rebuild or a new vehicle as soon as voluminous plumes of blue smoke start billowing out your tailpipe — which won't be long from now.

If the valve train looks reasonably clean, then you should thank your lucky stars, change the oil and set a recurring event in your calendar to change the oil every six months. Then set about 15 or 20 reminders so you can't ignore it.

A: First, you figure out the difference between the hybrid and non-hybrid version of the same car, equipped the same way. Let's say there's a difference of $2,400. Now the question is, how long will it take you to save $2,400 in gasoline?

To find that, go to fueleconomy.gov and look up the average miles per gallon of each vehicle. Take the number of miles you drive in an average year and divide it by each of those mpg numbers. That tells you how many gallons of fuel you'd need to buy in a year. Finally, multiply the number of gallons you'd buy in a year by the price of a gallon.

Now you can put it all together. After figuring out how much gas money you'd save in a year, you can extrapolate how many years it would take you to "pay off" the $2,400. Let's say it's about five years. So if you keep your cars for six or more years, it's worth it. If you keep your car for three or four years, it's probably not.

Now, there are other variables. For instance, hybrids use regenerative braking, so you'll spend less on brake pads and rotors. You'll also be using the gasoline engine less, which will lower the cost of maintenance (oils, fluids, belts and filters).

And some people believe there's value in decreasing pollution. We don't have the formula for that.