Why Oil Matters

When it comes to routine maintenance, changing your oil is one of the more inexpensive services. Despite its relatively low cost, oil plays a vital, multifaceted role in your engine, and spending a little money on routine oil changes can help prevent you from forking over large sums of cash down the road.

The Role of Oil

Engine oil provides three key functions in a modern internal combustion engine, according to Dennis Bachelder, a senior engineer at the American Petroleum Institute. First of all, it helps keep engine components working smoothly together.

"It's typically a lubricant to keep the moving parts separated by an oil film," Bachelder said.

The inside of an engine — a place where there are thousands of controlled detonations happening every minute — can be a hot place, and oil helps draw heat away from the combustion chamber, Bachelder said. Lastly, it helps prevent carbon and varnishes from accumulating in the engine.

"The detergent dispersant keeps the engine parts clean," Bachelder said.

What Oil Should I Use?

There are many types of engine oil, but it's fairly easy to determine what kind you should use in your car. Your owner's manual should tell you the appropriate viscosity of oil — like 5W-30, for instance — that your engine requires, and the viscosity may also be stamped on the oil reservoir cap on the engine itself.

There's more to oil, though, than just viscosity. The API certifies oil based on performance criteria determined by automakers, engine builders and oil producers. Oils that meet the standards receive the API Certification Mark, which should be easy to see on an oil container.

As of publication, the highest API oil category for gas engines is SM. It's designed for all gas engines, and, according to the API, oils with this rating offer "improved oxidation resistance, improved deposit protection, better wear protection, and better low-temperature performance over the life of the oil." Other gas-engine categories in use include SL, which is designed for 2004 and older engines, and SJ, which is for 2001 and older ones.

Bachelder encourages consumers to know what kind of oil is being poured into their car during an oil change.

"You can find that you will get an inferior oil and you won't even know it," he said.

In addition, he says consumers need to be wary of off-brand motor oils that tout the API's SA category. This is an obsolete designation that shouldn't be used in cars built after 1930. There are no specifications for SA oil, so there's no way to know what's in it.

How Often Should I Change My Oil?

Oil changes are often recommended when a certain mileage or time limit is reached. The most familiar interval is 3,000 miles or three months, but what's more important — the mileage or the time?

"Typically it's always the mileage that dictates when an engine's oil needs to be changed," said Ray McDonald, global technical adviser for Mobil 1. "You bump up against the mileage limit before you hit the time limit."

"Personally, I would say that the mileage has more impact than the time. In my experience that's the way it works," Bachelder said. "When the car is sitting the oil is not really changing."

Oil-change intervals used to be much more frequent than they are today. According to Dewey Szemenyei, chair of SAE International's engine lubrication committee, 50 years ago you would have had to change your oil every 500 miles. Today, the drain intervals for some oils dwarf even the 3,000-mile standby.

McDonald says Mobil 1 Extended Performance oil has a 15,000-mile or one-year service interval, but cautions that you should still follow manufacturer oil-change recommendations for your car. Meanwhile, AMSOIL's Signature Series 0W-30 oil has a 35,000-mile or one-year service interval.

"We've always focused on extended drain intervals," said Ed Newman, director of advertising at AMSOIL. "That's an environmental benefit."

Where and how you drive can influence how often you need to change your oil. Szemenyei said factors include extreme heat, towing a trailer, and driving where there's dust and sand.

What if I Don't Change My Oil?

If you're reading this article, you're probably not one to skip a few oil changes, but you might be wondering what might happen to your car's engine if you did. In short, it's not pretty.

One of the problems, according to Bachelder, is that there will begin to be build-up in the cooler parts of the engine, like the crankcase and around the camshafts and valves.

"You would get the carbon deposits, the sludge as you would call it, coming out" of the old oil, he said. "To clean the engine would be relatively expensive because it would require taking parts off."

"It can get to the point where it's so thick that it can't be pumped," McDonald said. "You can start to wear out your piston rings. Once those begin to wear it's not easy and not inexpensive to fix that problem."

It can get worse. Bachelder says the pistons could eventually seize or the camshafts could be damaged. Getting a bill for either one of these repairs would make even a few high-quality oil changes look like pocket change. Regular oil changes, according to Szemenyei, are like "really cheap insurance" for your engine.