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What Does Motor Oil Do?

Updated: Mar. 02, 2023

Get smart about car oil

car-oilFamily Handyman
Learn what's new in motor oil (a lot!), what oil does and doesn't do, how often you need to change it and how the right type can help your engine's performance.

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The importance of car oil

Car oil has gotten much better over the years. And, with car oil-life indicators on newer cars, drivers don’t have to guess when to change their oil. Yet even with better oil and oil change reminder lights, repair shops are reporting a shocking increase in the number of engines damaged by wear and sludge buildup.

It turns out that many DIYers and even some oil change shops are using the wrong car oil, and drivers are simply going too long between oil changes. Just as troubling, many drivers have almost completely abandoned the job of checking their car oil level when gassing up, and many more are causing engine damage by driving with low oil levels. Too many drivers seem to follow their own ideas about which oil they can use and how long they can run it in their engine. But that leads to trouble. Whether you change your own oil or take your vehicle to a shop, you need factual information right from the experts. Buckle up. It’s time to get smart about oil.

Car Oil: It’s more than just viscosity

Photo 1: Find the engine-specific car oil

The label under the hood of this VW says to use only oil that complies with VW specification 502.00. To find oil that meets that standard, look for the specification on the oil bottle label.

Every owner’s manual lists the recommended oil viscosity for the engine. But knowing the correct viscosity is just the starting point. Your car may also require synthetic oil or oil that meets a specific industry service rating (API and ILSAC are two examples). The most recent ratings are API-SN and ILSAC GF-5. Both are backwards-compatible, so if your manual shows a lower rating, you can safely substitute the newer oil.

But not all carmakers use those two industry rating services. European carmakers may require a regular ACEA-rated oil in some engines and a different oil that meets more rigid specifications in another engine. Manufacturer-specific and engine specific oil is a growing trend, and when a carmaker specifies a particular oil for an engine, that’s the only oil you can use (Photo 1).

It really comes down to this: Engine designers know more than you or your buddies. If you use the wrong car oil, you can destroy your engine. The damage won’t be covered by the factory power train warranty or your extended warranty, even if the oil changes have been done by a pro.

Where to Buy Car Oil

If you use a shop for oil changes, make sure it has the correct oil on hand. The shop may charge more for the special oil, but it’ll keep your engine running longer. When changing your own oil, you may have trouble finding the proper oil at big box stores, but you can always get it (or order it) at any auto parts store.

Should you use synthetic car oil?

If your car doesn’t require synthetic oil, here’s why you should switch to it anyway. Synthetic oil is made from natural gas or crude-oil feed stocks that go through a chemical reaction that results in uniformly sized molecules. The uniform size reduces friction, heat and wear in your engine. Name-brand synthetic oil has higher-quality and longer-lasting additives that keep your engine cleaner. And, since it doesn’t contain paraffin (wax) like conventional oil, it flows faster and builds pressure faster on cold starts. Sure, it costs a couple bucks more per quart, but it’s a far better lubricant. It’s worth the extra cost, especially if you love your car and/or plan to keep it for years to come.

Oil-Changing Tips

  • If the engine is cold, start it and let it run for five minutes to warm the oil. If it’s hot, wait at least 30 minutes to avoid getting burned.
  • Never use an adjustable wrench or socket on the drain plug. Use the proper size box-end wrench, usually metric, for the plug.
  • Always use jack stands. Never work under a car that’s supported by a jack only.
  • Use new oil to coat the oil filter gasket before spinning it on.
  • Always hand-tighten the filter. Never use a filter wrench.
  • Find an oil/oil filter recycling center near you (see below).
  • Line up all the oil bottles you’ll need for the fill so you don’t lose count as you pour.

What about oil for older cars?

Photo 2: Older engines need stronger medicine

Fill older engines with high-mileage synthetic oil to keep them running cleaner and provide maximum protection against wear.

You can argue that your old car racked up 100,000 miles just fine with conventional car oil. Great. But now it has some wear on the piston rings, and it’s generating more “blow-by” (combustion gas slipping by the piston rings). That increase in blow-by means more acid, soot, corrosion, varnish and sludge formation throughout the engine. That’s precisely why switching to synthetic oil makes so much sense for older cars. The more robust additive package is especially well suited to keeping an older engine cleaner and running longer. If you change your own oil, switching to synthetic costs only about $10 per change. We think it’s well worth the extra cost.

Internet lore advises performing an engine flush before switching to synthetic. That’s a horrible idea. Just drain the old oil and remove the oil filter. Then pop on a premium filter and pour in the synthetic oil. And it’s time to discontinue the age-old practice of adding a higher-viscosity oil to combat low oil pressure. Higher-viscosity oil actually increases friction and reduces flow rate, causing the oil (and engine) to run hotter. So you wind up with lower gas mileage, more wear and sludge buildup.

High-mileage synthetic oil contains film-strengthening additives to improve ring sealing and oil pressure. Many brands also include seal conditioners to soften stiff seals, as well as extra anti oxidant, anticorrosion, antiwear and detergent additives to handle the crud in the crankcase. So, if you have more than 100,000 miles on your engine, switch to a high-mileage oil. It costs only about $1 per quart more than regular synthetic.

Buy the Right Oil Filter

Select an oil filter that’s designed to last as long as your oil change interval. Economy oil filters last about 3,000 miles. So if your change interval is 7,000 miles, it won’t do its job for the last 4,000 miles. To get a filter that’ll last as long as your oil, plan to spend at least $6. Look for terms like “extended performance” or “extended life” on the box.

When should you change car oil?

Everybody’s full of advice about how long you can go between oil changes. But unless they know your driving habits, that advice is just hot air. Frequent cold starts, short trips (less than 4 miles), stop-and-go driving, hauling heavy loads and lead-foot starts are incredibly hard on oil and deplete the additives quickly. Your owner’s manual lists a different oil change schedule for this kind of severe driving, a category that includes most drivers. Unfortunately, those same drivers change their oil according to the optimistic “normal” schedule. So follow the oil change schedule that applies to your driving style. Or, if you have a newer vehicle with an oil-life monitor, rely on that.

Are You a Severe Driver?

Believe it or not, most people fall into the severe driving classification. The Center for Auto Safety says this:

Extended oil change intervals of 7,500 or 10,000 miles or more are based on ideal operating conditions, not the type of short-trip, stop-and-go driving that is typical for many motorists. Consequently, most drivers should follow a “severe” service maintenance schedule rather than a “normal” one to protect their engines.

Severe driving includes:

  • Most trips are less than 4 miles.
  • Most trips are less than 10 miles when outside temperatures remain below freezing.
  • Prolonged high-speed driving during hot weather.
  • Idling for extended periods and continued low-speed operation (as when driving in stop-and-go traffic).
  • Towing a trailer.
  • Driving in dusty or heavily polluted areas.

Some engines, such as diesels, suffer more blow-by than others and typically require more frequent oil and filter changes. For most passenger cars and light-truck diesels, the oil should be changed every 3,000 miles without exception—especially in turbo diesels.

Turbocharged gasoline engines also require more frequent oil changes because of the high temperatures inside the turbo, which can oxidize oil. A 3,000-mile oil change interval is also recommended for all turbocharged gasoline engines.

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Trust the car oil change light if you have one

Some vehicles have an oil change reminder light that turns on when you’ve reached a set mileage. Those systems don’t take your driving habits into account. So you have to adjust your oil change intervals according to how you drive. However, oil-life monitoring systems do track your driving habits. The computer records the number of cold starts, ambient and engine temperatures during startup, driving time between starts, engine load and whether the miles are highway or stop-and-go. It runs that data through an algorithm to estimate the remaining oil life. If you do a lot of short-trip city driving, the light will come on sooner than if you make long commutes.

You can trust the carmakers’ oil-life monitoring systems only if you use the recommended oil and don’t use any aftermarket additives. Pour in some miracle oil stabilizer and all bets are off.

What about those long-distance motor oils?

What about those claims that certain oils can go 15,000 or 25,000 miles between changes? Is there really an oil that comes with a 300,000-mile engine guarantee? Because severe driving degrades oil additives faster, oil manufacturers often qualify their oil change recommendations in the fine print. In some cases, your driving habits may require you to reduce the advertised mileage interval by as much as half. So don’t assume you qualify for the best-case scenario: Read the instructions and the warranty terms. And no matter what the companies’ promises are, if you skip the carmakers’ recommended change intervals, your factory or extended warranty will be void. So you’d have to depend on the oil manufacturer to pay for repairs.

How to Protect Your Warranty

If you do your own oil changes and want to maintain your factory or after-market warranty, you’ll have to prove you changed your oil on time. That’s easy to document.

If you have a shop do your oil changes, make sure the receipt shows the oil viscosity and service rating.

Check your car oil level

Photo 3: Check the oil level every 500 miles

Add more oil to bring the level up to the FULL line, even if the dipstick shows the oil is down only a half quart. This will prevent rapid oil degradation.

Photo 4: Overfilling damages the oil and engine

Overfilling creates foam, which rapidly degrades the oil and causes additional engine wear. Foam doesn’t remove heat as efficiently as liquid oil, so the engine and the oil get much hotter. The extra heat and air rapidly degrade the oil.

Unless you own a luxury vehicle equipped with an oil level sensor, your vehicle won’t tell you when it’s low on oil because its warning light only indicates oil pressure. By the time the warning light comes on, you’re already dangerously low on (or completely out of) oil.

Here’s the bottom line: All engines burn oil. And with longer intervals between oil changes, you can count on losing up to a full quart before it’s time for your next oil change. Driving a quart low puts tremendous stress on the remaining oil, dramatically reducing its useful life. So get into the habit of checking the dipstick every other fill-up. Add oil to raise the level to the full mark, even if it’s down just a half quart (Photo 3). But don’t overfill (Photo 4).

Save Money on Car Oil

Buying oil by the jug saves about $17 per oil change. Pour the used oil into an empty jug and drop it at a recycling center. To find the nearest recycling center for used motor oil, search “oil recycling” with your zip code. Auto parts stores also either offer the service or can tell you where to go.