Rick's Free Auto Repair Advice

Change oil based on time or mileage

Do you change oil based on time or mileage?

If you check your car owner’s manual you’ll see the carmaker wants you to change oil based on time and mileage. If you’re not accumulating a lot of miles, you’re probably wondering why the carmaker imposes a time limit on oil that hasn’t come close to its maximum mileage limit. That begs the question; does oil get old by sitting in your engine? And the answer is, yes, it does. But, of course, it’s not that simple.

Fresh oil doesn’t get old by sitting

If you have unopened oil in your garage that’s been sitting for five years and that oil meets the API rating specified by your carmaker (like SL or SM), you can use it in your engine. New oil doesn’t go bad from sitting in an unopened container.

Used oil DOES go bad from sitting in your crankcase

That’s a pretty important statement: oil goes bad from sitting in your crankcase. That’s why all carmakers tell you to change oil based on time or mileage.

On every cold start, you’re adding fuel and water to the crankcase and the crank movement is whipping air into the fuel, water, and oil. That causes the oil to oxidize and thicken. Thickened oil that contains water and fuel develops an acidic pH and forms sludge. Remember that for later.

Here’s why oil goes bad from sitting in your engine.

1) Oil dilution: When you start a cold engine, the computer provides a rich air/fuel mixture to overcome the “quenching” effect of the cold engine block. Remember, you’re trying to start a fire in a cold engine and that cold metal tries to put out the fire. Fuel is a solvent. So, the instant you inject cold fuel into the combustion chamber, you wash a certain amount of oil off the cylinder walls, piston head, and rings. At startup, some of the gas/oil mix gets blown past the piston ring gaps and into the crankcase where it mixes with the oil in the sump.

2) Water formation: Water is one of the byproducts of combustion and it too is pushed past the piston ring gaps into the crankcase where it mixes with the remaining oil.

3) Oxidation. When the components in oil come in contact with oxygen and heat, free-radicals react with oxygen to form peroxy free-radicals which attack the hydrocarbons to form hydroperoxides which then decompose into aldehydes, ketones, carboxylic acids, and other oxygen-containing hydrocarbons. The oxygen compounds polymerize to form viscous soluble materials (lubricant thickening) and insoluble materials (sludge and deposits.)

4) Corrosion. Oil contains anti-corrosion additives to protect against rust formation. Water and acids in the oil deplete the anti-corrosion additves over time.

Oil oxidation and short trips and stop and go driving

You’ve started your cold engine and now you take a short trip or you drive to work in stop and go traffic. As you drive, the oil heats up and that helps evaporate off some of the raw fuel and water. That’s a good thing. But adding heat to oil, fuel, water combination also increases the rate of oxidation. That’s a bad thing. If you drive short trips or drive in stop-and-go traffic, you accelerate the oxidation rate of your oil. Remember that for later. Longer trips at highway speeds, on the other hand, help remove the fuel and water from the crankcase oil. But the longer driving still oxidizes the oil.

Oil contains additives to combat oxidation and fight off corrosion

Fresh oil contains anti-oxidants to reduce oxidation and anti-corrosion additives to protect the metals in your crankcase from corrosion caused by water and acids.

Carmakers recommend checking your oil level on a regular basis

All engines burn some oil, even new engines from the factory. As an engine burns oil, the reduced oil volume stresses the remaining oil. In other words, if you drive your car when it’s 1-qt low on oil, you wear out the anti-oxidation, anti-corrosion, detergent, and suspension additives in the remaining oil.

This is an important point to remember because every time online forums members start the fight over whether it’s ok to go beyond the carmaker’s oil change time limits, no one talks about whether the driver has checked and topped off the oil level. And, in reality, nobody checks their oil level these days. Ask any oil change outlet and they’ll tell you horror stories of how little oil drains from engines during a routine oil change.

Here’s where the change oil based on time or mileage recommendation comes into play

Whether you drive short distances, drive in stop and go traffic or drive long distances at highway speeds, if you let your oil sit for long periods, any fuel, acid, or water that’s left in the oil will continue to degrade the anti-oxidation and anti-corrosion additives in your oil.

This is no different than letting a bowl of soup sit exposed to open air on your kitchen counter for months. The air will react with the soup. I’m not suggesting that the oil in your car develops microbial growth like the soup on your counter. But the oil will continue to oxidize and corrode the metals in your engine. That degradation will be even worse if you haven’t checked the oil level and topped off as recommended because you’ll have accelerated the depletion of the anti-oxidation and anti-corrosive additives in the remaining oil.

The longer you let used oil sit in your crankcase, the more it degrades the oil’s additives and the more it oxidizes and corrodes the metals in your engine. That’s why carmakers want you to change oil based on time and mileage.

©, 2021 Rick Muscoplat


Posted on by Rick Muscoplat

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