Engine oil keeps moving parts from grinding against each other. It does more than reduce friction, though, and as it ages, it becomes less effective at protecting the engine.

Knowing what wears oil out helps explain why cars with similar mileage can be due for service at different times. It also helps to know what to check between changes.

<h3>What engine oil does</h3>

Oil does four things at once: it lubricates moving parts, carries heat away from hot spots, holds dirt and fine metal particles in suspension, and helps seal the gaps between the piston rings and cylinder walls. As the oil breaks down, it offers less protection against wear and deposits.

<h3>Why oil breaks down</h3>

Heat and oxygen gradually break down oil. Additives that reduce friction and neutralize acids get used up, and the oil can thicken or leave sludge in the oil pan. That buildup is different from the normal darkening you might see when you pull out the dipstick.

Short trips present a different problem. On a quick drive to the store, the engine may shut off before the oil gets hot enough for moisture to evaporate. When that happens repeatedly, condensation can accumulate and mix with the oil. A car can rack up very few miles this way.

<h3>When to change the oil</h3>

Check your owner's manual rather than relying on the sticker from the last shop. If the schedule gives both a time limit and a mileage limit, follow whichever comes first. If it lists a separate schedule for demanding conditions, check whether frequent short trips or towing put your car on it.

If your car has an oil-life monitor, follow the manual's instructions for using it. Some monitors estimate when service is due from how the car is driven, taking factors such as engine temperature and time spent idling into account. That estimate isn't a reading of how much oil is in the engine.

<h3>What an oil change costs</h3>

In the U.S., the American Automobile Association puts the cost of an oil change at $20 to $100, depending on the oil and where the work is done. Full synthetic generally costs more than conventional oil, but the vehicle's required grade and specifications should guide your choice.

When comparing shop quotes, check that they cover the oil your engine requires and a replacement filter. A timing chain replacement or an engine rebuild is a much bigger job than routine service. Switching to synthetic oil doesn't by itself mean you can extend the vehicle's service interval.

<h3>What the oil filter does</h3>

The filter catches grit and metal particles so they don't keep circulating. If the filter is restricted, a bypass valve can let oil flow around it to keep the engine supplied. The trade-off is that the oil reaches parts such as the bearings without being filtered.

Most manufacturers call for a new filter at each oil change. Check the schedule for your vehicle, and if a shop does the work, look for the filter on the invoice. If your car uses an oil-life monitor, make sure it was reset after the service.

<h3>Checking oil between changes</h3>

If your car has a dipstick, check it once a month. Park on level ground, turn off the engine, and follow the manual's instructions on when to take a reading. Pull the dipstick out, wipe it clean, push it all the way back in, then pull it out again.

If the level is at or below the minimum mark, add oil that meets the manual's specifications. Add a little at a time and recheck the level so you don't fill past the maximum mark. If the oil repeatedly drops to the minimum between changes, have the engine checked.

Dark oil alone doesn't mean you're overdue. A new ticking sound or a burning smell warrants a check rather than an assumption that fresh oil will fix it. If the oil pressure light comes on while the engine is running, pull over safely and shut off the engine. That's not a routine service reminder.

After your next oil change, keep the receipt and record the date and odometer reading together. If the shop sticker and your maintenance schedule don't agree, you'll have a record of the last service to work from.