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Q: I use full synthetic oil because it lasts more than twice the regular stuff and I read that it reduces wear due to lack of varnish build-up in the cylinder walls. What is your advice on the mileage and any added inputs on the subject? I understand that mileage could be as much as 15,000 except the filters need replacing, at least, around 10,000 miles.

A: First of all, synthetic oils don't "last longer" than petroleum lubricants. In fact, neither type actually wears out. What does occur over time and mileage is the consumption of components of the additive package and, more important to my way of thinking, contamination. Combustion byproducts, fuel, moisture and particulate debris accumulate in engine oil. Additives like detergents, dispersants, viscosity improvers, etc. are effectively "used up" during the lubricant's service life.

Full-flow oil filters, which filter all oil as it circulates through the engine, only trap solids roughly 20 microns or larger, meaning that smaller particulates continue to circulate in the oil. A micron is one-millionth of a meter. Putting this into perspective, a human hair is as small as 30 microns thick. While oil filters do a remarkable job of filtering debris, they cannot and do not filter out soluble contaminants like fuel and water.

Motor oil filters are required to balance filtration versus flow. Finer filtration — say, 10 microns or less — could lead to a lack of oil flow in cold weather causing potential engine failure.

I agree that chemically built synthetic lubricants provide better lubrication performance than petroleum oils, particularly in the extremes — cold starts, high-temperature operation, sustained high engine output conditions, etc. But only by a small margin. Carmakers publish their oil change interval requirements without distinguishing between natural and synthetic lubricants. Nor does the API service rating for motor oil. And yes, some carmakers call for the use of synthetic lubricants in their engines.

Add to the equation the fact that modern technology including design, electronics, metallurgy, manufacturing and assembly techniques deliver high-quality engines with much greater durability and reliability. It is no longer uncommon for automotive engines to deliver 150K, 200K or more reliable miles.

So yes, oil change intervals have lengthened significantly. But the fact that carmakers distinguish between "normal" and "severe service" operation confirms that lubrication qualities — not the oil itself — is influenced by time, mileage and operating conditions. That's why oil service monitoring systems calculate oil change recommendations rather than suggesting specific mileages.

On the other side of the issue is the simple fact I continue to focus on — it is my name on the title. I'm the one responsible for maintaining the vehicle — not the carmaker. I see my more frequent oil change intervals — roughly 5,000 miles — as a low-cost insurance policy to protect against potential engine problems.

According to the AAA, the average annual cost of owning, operating, fueling, depreciation, licensing, insurance and maintaining a late model automobile is approximately $9,000. Spending $50 four times a year for oil changes is, at most, about 2 percent of the total. Cheap insurance, for sure.

Motoring note

Pat Brown offered a great tip in response to last week's PT Cruiser question about dashboard warning lights coming on. "I had this same problem with the lights coming on and the speedometer crashing last summer and it is a real easy and free fix. If the vehicle has power seats like mine, push the button and to raise the seat off the floor just a bit. My son has a Dodge minivan and told me that he had the same problem with it, but it only happened when he drove it as he is tall and put the seat down to the bottom and back as far as it would go. When his wife drove, she moved the seat higher and closer to the steering wheel and had no problems."

Paul Brand is the author of "How to Repair Your Car" and "How to Repair Your Truck and SUV," published by Motorbooks.