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Monday, May 23, 2005

The Oil-change Conspiracy

I recently had this conversation, about oil change intervals, with a relative. She has been indoctrinated to change her oil every 3,000 miles by her auto dealer. I told her she was throwing away money.

Don't let the quickie-lube guy or the dealer fool you. Unless you are doing "hard duty" driving (lots of stop and go, or high temperature, for example), you do not need to change your oil any more frequently than 5,000 miles. If you drive a higher mileage car (like myself), and your car loses some oil, you should keep an eye on it and add some as necessary.

I learned this some years ago from Consumer Reports, which did an oil study on a fleet of NYC taxi cabs, probably one of the harshest use profiles there is. CR found no additional engine wear between 3,000 mile and 6,000 mile schedule taxis. They recommend oil changes on 7,500 mile intervals, for the average driver, unless the manufacturer recommends otherwise.

The bottom line. Modern motor oils needn't be changed as often as oils did years ago. More frequent oil changes won't hurt your car, but you could be spending money unnecessarily and adding to the nation's energy and oil-disposal problems.

Even in the severe driving conditions that a New York City taxi endures, we noted no benefit from changing the oil every 3,000 miles rather than every 6,000. If your driving falls into the "normal" service category, changing the oil every 7,500 miles (or at the automaker's suggested intervals) should certainly provide adequate protection. (We recommend changing the oil filter with each oil change.)

Tom and Ray Magliozzi advise an oil change interval of 5,000 miles.

Consider the savings. Over 5 years, a 3,000 mile oil change schedule will cost you about $500 on oil changes ($25/per x 4/year). A 5,000 mile schedule will cost you about $300, and a 7,500 mile interval will cost you $200.

I learned about another controversial practice from an old-timer. I'm not brave enough to try it, but he swore by it: he changed his oil filter every 3,000 miles religiously, but didn't ever drain out the oil. Just topped it off at every filter change. He swore that he never had an engine go bad with this method. He claimed that the gunk would leave with the filter, and the clean top-off would keep the additives fresh, but I suspect this is pushing it. My understanding is that oils (and additives) do break down, and mixing in 1 quart of fresh oil with 4 or 5 quarts of tired oil is not going to be as effective as all new oil.

Don't send any extra money to the Saudis. Go by what your owner's manual tells you, not what the quickie-lube and dealer say. They want to sell you the service.


John in Exeter NH said...

In 1972 I bought a new Road Runner with the 340CI V8/standard shift. I would run it mainly on the hi-way at 70-80 mph. I would change the oil and filter every 6000 miles. Between changes I would add a quart of 10W-30 Shell Fire and Ice. I would also use what ever filter was available (usualy Fram). Never had a problem. I had a 1992 Sentra that I ran for a year without changing the Mobil 1 I put in it but, did change the filter and top it off every 5000 miles. No problem there either. Now I change the oil/filter every 6 months. The Dodge dealer gave me lifetime oil/filter changes on the vehicle for $7.77 each visit. My other cars, I use synthetic Mobil 1 or Syntec 5w-30.

The Angry Engineer said...

If you're really worried about not "sending any extra money to the Saudis", then I'd suggest that fuel economy is the first place to look.

For a vehicle that runs nearly constantly like a taxi, I'd suggest that an extended oil drain interval is probably OK, despite the fact that it'd be considered "severe-duty" driving by the OEMs. Also take into consideration that taxis are built for severe duty (GM's Caprice 9C6 taxi package included the same cooling-system upgrades that were included on the 9C1 police package), and that a big V8 loping around at city speeds might actually be easier than a modern I4 or V6 on the highway, especially considering modern trends towards warmer thermostats.

I'd personally suggest sticking to the OEM's recommended schedule, although I generally don't panic if I go a thousand miles over on an oil change.

Anyone seriously interested in extended oil-change intervals should really invest in oil analysis, such as that offered by Blackstone Labs. I've only recently begun using such a service, but I've got some friends in the off-highway equipment repair industry that report it's very effective.

Real World Observation said...

One thing we learn in the real world, is that engineers are not the most brilliant persons on the planet,(unless you ask them), and those in the field are the ones who fix their mistakes. What about the toyota/lexus line of vehicle that are "gelling" (fancy for sludging) due to 5-7.5k mile oil changes? Also should be noted that 3month 3,000 mile service is for inspection and preventative maintenance on other items, tire pressure, belts, hoses, adjustments, fluid top off, and safety inspection to name just a few. This should be done every three months regardless of conditions. Also remember the oil change at 3,000 miles is not to say that the oil is bad at that point, but it is also a method of filtration, and is done before any build-up or breakdown gets a chance to occur. As far as waste... most facilities recycle used oils! Also the Gunk does not go out with the filter! This is foolish talk, most vehicles are equipped with a check valve that shuts the filter off at an rpm about double the idle! this means that the filter only works for a portion of the time, and field experience shows that most of the gunk settles in the cylinder head valleys, and the oil pan! Looks like, as in most cases, this is a false prophet, BEWARE!!!

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