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How often to change oil

How often to change oil

If you’re wondering how often to change oil, you must first understand the difference between normal driving and severe service driving. It’s all there in your owner’s manual. But we’ll go over it here.

There’s no one-size-fits-all mileage for when to change oil

Each carmaker has different recommended oil change intervals

image adding motor oil after oil change

for normal and severe service. Some engines with turbochargers require more frequent oil changes as often as every 3K or 4K miles.

Plus, the difference between normal service and severe service is often 50%; 6,000 for normal service or 3,000 for severe service. Toyota for example recommends oil changes every 10,000 on many of their vehicles. But if your driving meets the severe service definition. then Toyota recommends changing every 5,000 miles. If you follow the wrong oil change interval, you can void your warranty and damage your engine.

How often to change oil if you drive under severe service conditions

If you drive under any of the conditions listed below, you must change oil more often to prevent engine wear.

• Regularly driving short distance of less than 5 miles in normal temperatures OR less than 10 miles in freezing temperatures
• Extensive engine idling or low speed driving for long distances
• Frequently driving in stop-and-go conditions
• Driving in heavy traffic area over 90°F
• Driving as a patrol car, taxi, other commercial use (Door-Dash/delivery service) or vehicle towing
• Driving on uphill, downhill, or mountain road
• Towing a Trailer, or using a camper, or roof rack
• Driving on rough, dusty, muddy, unpaved, graveled or salt- spread roads
• Driving in areas using salt or other corrosive materials or in very cold weather
• Driving in sandy areas

If you drive under and of these conditions, follow the severe service oil change intervals listed in your owner’s maintenance guide.

Time between oil changes also is a factor

When it comes to fluid service, most maintenance guides list both a time and mileage interval. Why is time in service so important? Because the anti-corrosion, and pH balancing additives in motor oil start to deteriorate once they’re put into service. Carmakers know how long these critical additives last, even if you haven’t racked up enough miles to justify changing the fluid. That’s why fluids must be changed on time or mileage, whichever comes first.

Here are the most common motor oil additives, what they do, and how they degrade with miles and time


Exposure to oxygen causes the base oil to break down and form acids and sludge in your engine. Oxidation happens at all temperatures, but accelerates at when your engine is fully warmed up. Oxidation also increases in the presence of  water, wear metals and combustion byproducts present in blow-by gasses (air, fuel, and exhaust that seeps past the piston rings and into the crankcase).

Anti-oxidants (oxidation inhibitors) are used to extend the operating life of motor oil. But they are sacrificial additives, i.e. they are consumed while performing their duty

Rust and Corrosion Inhibitors

Rust and corrosion inhibiting additives work by neutralizing the acids that form in the crankcase and by providing a protective chemical barrier on metal surfaces to repel moisture. Once these additives are exhausted, corrosion sets in.

Viscosity Index Improvers

All motor oil thins as it heats up. To reduce thinning, all motor oils contain Viscosity Index Improvers (VII) made from a “watch spring” type of polymer that “unwinds” as it heats up. So the VII takes up more space, reducing the oil’s tendency to thin. As the oil cools, the VII molecules “wind back up” to help the oil flow better. VII is what allows an oil to be have different viscosities at different temperatures, like 5W-30.

However, VII molecules degrade when exposed to high loads in gears and bearings. The “squishing” forces in gears and bearings can permanently deform the molecule or cut into smaller pieces. At that point, the additive can no longer perform its intended function. So the oil becomes too thin at normal operating temperature, providing less protection against wear.

Like other motor oil additives, VII is sacrificial; it gets damaged while doing its job.

Anti-wear Additives

Anti-wear additives are activated by the high heat caused when metal-to-metal contact occurs. The anti-wear additives “melt out of suspension” and react chemically with the metal surfaces to form a film that minimizes wear.  They also help protect the base oil from oxidation and the metal from damage by corrosive acids. Like most other additives, the anti-wear additives get used up the more you drive.

Extreme Pressure (EP) Additives

EP additives chemically react with metal (iron) surfaces to form a sacrificial surface film that prevents metal parts from welding together during periods of metal-to-metal contact.

Detergent Additives

Detergents prevent deposits from forming on metal components and they neutralize acids that form in the oil.

Dispersant Additives

Dispersant additives keep soot particles suspended in the oil so they can be captured by the oil filter and not settle out of suspension to form sludge deposits.

Anti-foaming Additives

Rotating engine parts whip air in the oil. Foamed oil can’t carry out its job of removing heat from high friction areas. So anti-foaming agents reduce oil’s surface tension causing the bubbles burst. Because anti-foaming agents reduce foaming, they also reduce oxidation that occurs when oil bubbles carries oxygen around the engine.

Friction Modifiers

Friction modifiers help make engine oil more slippery to reduce friction and improve fuel economy.


All blow-by gasses contain water (by-product of combustion) which winds up in crankcase oil. When oil and water mix, they form a thick emulsion that begins the process of sludge formation. A demulsifier additive prevents the formation emulsion by separating the water from the oil.

Tackifier Additives

Tackifier additives prevent the oil from flinging off of the metal surfaces they’re supposed to protect.

Anti-oxidants, rust and corrosion inhibitors, detergent, dispersant and demulsifier additives work 24/7 to protect your engine and they all degrade once they’re exposed to oxygen, heat, fuel, and water.

Additional considerations for oil change intervals

The carmaker assumes you’ve used the recommended type of oil and the recommended oil viscosity. If you’ve used anything else, you can no longer follow the carmakers oil change interval recommendations. ALSO, the carmaker assumes you check your oil level and top off when you’re a half quart or more below the full mark. If you don’t check your oil and top off, all bets are off. Driving when you’re 1-quart or more below the full mark can reduce your oil life by at least 25%. The longer you drive on a reduced oil level, the more you wear out the oil in your engine.

Whether you check oil level and top off also determines when to change it

The oil change recommendations are based on the assumption that image showing checking oil level on you’ve followed the directions in your owner’s manual to periodically check the oil level and top it off with the recommended oil when it’s a half quart low or more. Most drivers don’t do that. That’s a huge mistake. Driving on low oil dramatically reduces the life of the remaining oil. For example, driving while 1-qt low reduces the life of the remaining oil by at least 25%.

If you don’t check oil level and top off, you can no longer trust your oil life monitor; it’s based on driving with the correct amount of oil.

©, 2020 Rick Muscoplat


Posted on by Rick Muscoplat

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