Oil Change - So How Often Should You Change It?

Most automobile owners manuals say to change the oil every 7,500 miles under normal driving. Synthetic oil can go 15-20,000 miles before any oil breakdown takes place. My dad used synthetic oil in his new 18 wheelers and changed the oil at 100,000 miles. He never had major engine failure.

Car and Driver tested New York taxi cabs under a test that changed oil at 6,000 mile intervals and found the oil to be near perfect after 6,000 miles of taxi service.

In a free market economy like ours, money rules. Don't fall for these promotion gimmicks. Ted D. AL

Dear Ted,

You are absolutely right. The 3,000 mile campaign was the brain child of Jiffy Lube. I like the idea of having the customer visit my shop every 3,000 miles. Sure, an obvious reason is a small sale for the oil change, but the real important factor is to have drivers routinely visiting a mechanic every 3,000 miles or three months to do a check up on their cars.

Most drivers don't even open the hood. And 3,000 miles for most drivers really means 4,000 miles when they actually come in to do it...they thought about it at 3,000 miles. I see the consequences every single day--a tire that is showing steel, a water hose swollen and about to bust, a leaking radiator, expired state inspection stickers, etc., etc., etc.

My point is: The average American driver drives 12,000 miles a year, so that is four oil changes a year. At $25 an oil change, that's about $100 a year to have your cars looked over and make sure things are A-OK. That is just a little bit more than a tow bill...the alternate price you'd pay if that radiator hose bursts on you in the summer, leaving you on the side of the road with a car load of kids and groceries.

So, with that said...it is a small price to pay for a well-maintained vehicle, a clean engine, and a little added insurance.

I do have a copy of the Motor Oil Bible ebook that you might find interesting.

Blessings, and thanks for your email.

Austin C. Davis
The Honest Mechanic

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Reader Feedback

Austin ,

I read with interest your reader's remarks and your reply and you

have both missed some important concepts, to wit:

1) Regular oil is poly-unsaturated, that is, it combines with other engine chemicals resulting from the combustion of gas and with atmospheric elements to produce a new compound which, eventually, if left in the engine long enough for the detergent additives to dissipate, will coat the engine with sludge.

Synthetic oil is saturated, will not combine with other available compounds and, unless contaminated, essentially lasts forever. Thus, the user of regular oil should change his oil before the modification of his oil through chemical reaction produces a new compound that lubricates poorly and the filter becomes clogged with the newly compounded sludge. My personal experience suggests 3500 miles as a point when significant alteration of the oil should be considered.

2) If synthetic oil is used, the oil's properties (viscosity, pouring temperatures, etc.) are determined by the actual molecular structure of the oil not by the addition of viscosity modifiers.

The main problem then becomes suspended dirt, thus synthetic oil requires changing to eliminate the slowly accumulating liquid sandpaper that will eventually damage an engine. Essentially, synthetic oil, again, would last, effectively, forever if it were not for the problem of particulate contamination.

3) Common filters might filter down to 4 microns, a size that is close to the bearing gap size in modern engines. As it becomes clogged with larger particles the filter begins to 'load up', decreasing its passage size and actually increasing its filtering quality until the filter throughput decreases and begins to starve the engine whereupon, hopefully (and unfortunately), the filter bypass activates and lets dirty oil pass through unfiltered.

Again, leaving the filter in the car for 15,000 miles as per some current car specifications is asking for trouble, especially in environments with high particulate density (smoggy cities, dusty plains, areas prone to tornados or high water levels....). 7,500 miles is a max for the filter and certainly for standard oil.

4) Extended oil changes can be effected if:  The oil chosen does not break down during the period of usage - spec: Synthetic Oil

The filter in use will not 'load up' over the period of use and is capable of filtering down to a particle size smaller than the bearing gaps of modern engines. Only the Amsoil bypass oil filter with its 1 micron filtering capability or similar quality unit would be adequate for this specification, however, it is not available for all cars and requires a bypass installation.

Solution: Use synthetic oil. Change it at 7,500 miles and change the filter halfway between changes.

Personally, my BMW specifies 15,000 mile oil changes and BMW insists on the use of synthetic oil in their products. If they could guarantee that the filter would remove all particles larger than the bearing gaps for that period I might comply.

As this cannot be guaranteed, I change my synthetic oil at 5000 miles to decrease the potential wear caused by suspended dirt, not because the oil wears out. What maintenance is done on a car during its lifetime will greatly determine the length of that lifetime. The gentleman who claimed he used standard oil for 100,000 miles in his truck probably halved the life of the truck's engine.

Car manufacturers can specify longer oil changes as the original owners will, statistically, sell the car long before the abused engine begins to falter and won't complain. Modern engines are built with significantly greater precision, closer bearing tolerances and better materials than ever.

Please note that, obviously, I'm a proponent of the use of synthetic oil, but only because of the superiority of protection and product longevity. The man who chose synthetic oil for this truck and left it in for 100,000 miles chose the right product but left it in his truck only considering the premise that it would not break down.

He was correct, but did not consider the consequences of particulate contamination verses the ability of the oil filter's capability to control the level of that contamination. Eventually, his oil would have a high level of destructive particles that were not contained by his filter. Hmmm...I wonder if he even changed his filter? Maybe he did change it regularly; maybe he actually was using a supplemental filter system... If he had used a bypass filter with a 1 micron filter then, probably, his 100,000 oil would still by 'happy' today.

Synthetic oil companies are not going to tell their customers that the main problem with the use of their product is contamination. Their advertisements that the oil is good for 25,000 miles leaves out other factors that govern change intervals. It should be, "Our product is good for 25,000 miles providing the user can figure out a way to control particulate contamination that will occur over this 25,000 miles better than that offered by the mediocre capabilities of the majority of engine oil filters".

Perhaps, 7.500 miles would be a more honest claim as they could match the cost of regular oil with 3,500 mile changes, yet offer greater engine protection and longevity.

Obvious #2 - I have very little respect for those companies that train consumers in faux facts.

Statement: "Modern engines built better and thus can tolerate longer oil change intervals."

Reality: Modern engines have much more accurate and smaller bearing gaps and, thus, it is even more important to maintain clean oil to preserve these fine tolerances. Modern engines produce more power per liter and, thus, more stress on those bearing and heat to be dissipated by the oil going through those bearings. The oil is responsible for about 30% of the heat dissipation and needs to have superior resistance to breakdown caused by stress and heat.

Allowing regular oil to remain in the engine through greatly extended change intervals under increased stress conditions only increases the potential for engine damage as the oil degrades, loses viscosity rating and its ability to perform under that heat and stress. Of course, as the engines are, indeed, better build, it takes longer for them to exhibit bearing or cylinder wear from poorly maintained oil, long enough for the initial buyer to have sold the car to the next hapless user.

Statement (from a Honda dealer): "Synthetic oil causes damage in our

cars and we don't recommend it."

Reality: Users who use synthetic oil think, as a result of oil company propaganda, that the car does not have to have its oil changed as often and simply ignore the 3-5,000 mile oil change recommendation, coming in to Honda at 15,000 mile intervals for their warranty checkups (and an oil change). This costs the dealer money as no service charges to the owner are rendered over that period.

The dealers are not, in addition, in a position to find other warranty problems at a point when their repair cost might be less. This is a legitimate complaint, but the solution is not to recommend an oil that requires frequent changes to get the customers into their shops, rather to educate the owners to bring their cars in for service when problems are suspected or offer warranty checkups at shorter intervals.

Keep up the good work on your interesting site.

Regards,

Larry

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Mr. Davis,

Thanks for your e-mail. There is a lot that can happen to even to a new vehicle. If not checked out, oil changes and the lot are very important. Case in point, my brother-in-law just purchased a brand new car and decided to take a trip with the family.

I offered (for free), to give the car a once-over just to be safe and make sure everything would be ok. He turned my offer down, and when he got into Ohio he felt a shimmy and stopped to have the car checked out.

When they went to pull the car into the garage, the front U-joint snapped. All that was said, a defective part. A few weeks later the car engine caught on fire and totaled the thing out. Moral of story? Anything can happen to any vehicle , new or old. I have been a mechanic for over 20 yrs. Now, and I have seen some really scary things over the years. Money or not...if you don't take care of it, it will cost a lot more down the road.

Thanks for your time,

Tony

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