When Is Midnight for Your Car’s Oil?

Question from Carol D.:

(Above): On the dash of the Mini. Q. For these newfangled cars, which include my Mini, the car tells you when you need your oil changed. It worked out that I was able to get about 13,000 miles be­fore

the first oil change. They keep telling me at the Mini dealership that because the oil is synthetic, the car can go longer between oil changes. I really doubted this, but they insisted it was O.K.. So, I guess my question is, how long can you go between oil changes when your car uses synthetic oil? I also wonder how your car “tells” you when you need an oil change. What type of monitoring is it do­ing?

A. Trust the maintenance mind­er. Much has changed in engine design since the days of 3,000-mile oil changes. Cylinder liners, how the oil is circulated inside the engine, and filter design have evolved. Also, if the car carries more oil, it makes sense that it might last longer.

(Above): The maintenance minder on the Highlander. Your little Mini (redundant?) engine has a capacity of nearly 5 quarts. I’ve owned older, much larger engines that held less.

The fact that the oil is syn­thetic is a big part of why the car can go so long. Synthetic oil is designed and proven to work past 10,000 miles. Synthetic oil is simply a more highly engineered liquid. The molecule size is more closely controlled than mineral oils and contains fewer contami­nants.

Synthetic oil is tested at high and low temperatures to ensure it exceeds the parameters that oils are expected to meet. The price tag proves it!

Synthetic oils became popu­lar with high-performance cars. I looked at the web site for the most popular of the brands, and the cars that come standard with synthetic oils tend to be per­formance oriented such as the Dodge Viper. However, synthetic oil is making its way into more main­stream vehicles as turbocharging becomes more popular. Turbo­chargers have bearings that ro­tate at up to 20,000 revolutions per minute (RPMs). They can require some special lubrication techniques to live as long as the rest of the engine will. One reason not to change oil more than needed is botched jobs. I’ve had improperly re­placed oil filters leak on my cars. Folks I know have had the me­chanic strip the threads that are in the oil pan, which requires an ugly fix. Mini does recommend you change the oil at least every two years even if you have very few miles driven.

As to how the car knows when you need the oil changed, that depends on the specific car. Some use an algorithm. The car’s computer tallies things like revs, cold starts, and how much time you spend in the high-RPM range and then calculates how long you can go before maintenance. Be­cause of this, you might notice that number in “Miles Left Before Maintenance” counts down dif­ferently from the actual driven miles as you go along. The car is making a projection based on how you have driven already. If you drive on the highway a lot, you will go more miles between maintenance reminders than if you frequently race around town on short trips. Other cars, like my Toyota Highlander, simply count the miles and start to remind you when it gets to the set point — 5,000 miles in the case of my Highlander. Let me restate that the point is not to try to find some uni­versal oil change mileage for all cars. Rather, the manufacturers should be a trusted guide for all of the maintenance intervals for the cars they make. Some cars might still require a 3,000-mile oil change. Many still recom­mend 5,000. You need to check the manufacturer’s manual. Think of it this way. How often should you feed your pet? It sort of depends on the pet, wouldn’t you agree? Cars are this way. You can’t come up with a single rule of thumb to cover them all. They are just too different from model to model. It is wise to check the oil and other fluids in any car at least monthly. Sometimes oil is con­sumed and needs to be topped off. Be sure to read the manual. Commonly checked fluids include coolant, brake fluid, transmission fluid, and, in some all-wheel-drive vehicles, differential fluid. The list is vehicle-dependent. You need to read the manual.

Some fluids are meant to be checked cold and some when the car is hot. I’ve not found this to be completely consistent from car to car and fluid to fluid. Please send me a question at john@columnshift.com.