I just got this e-mail from a friend. It was his first oil change on a new 2003 Chevrolet Tahoe. He wondered how the vehicle could go so far without the oil monitor light going on.

He wrote, "I've never gone this far on an initial oil change before. Usually changed that oil out at 1500-2000 miles in the past. I had 5500 miles on the truck and it still had 23% of its life left in the oil." That calculates out to 7142 miles before it would have told him to change the oil, and 4.25 months (or 17 weeks) to reach the 7142 miles. He was curious about whether GM was lengthening the algorithm without telling consumers what they were doing.

Naturally I was curious to learn more and thought it a good opportunity to revisit an issue which five years ago seemed to put shudders into a lot of quick lube owners and operators. Did GM change the algorithm?

THE ISSUE
The matter came to his attention because the oil change light in his 2002 Chevrolet Tahoe came on much more often, even though it was the same engine, same drive train, and same travel route to work each day.

"Driving the same route, during spring and summer weather, my oil change light came on once at 3800 miles, and once at 4200 miles on my 2002 Tahoe. All things being equal, one would have thought that the severity of my driving during the colder months in the newer truck would have resulted in even shorter drain intervals as compared to my old one," he wrote.

I told him I'd be going to the SEMA Show in Las Vegas and would make it a point to find someone from GM who could address this issue. I also did some Internet research to see what else was out there.

When we talked later, I learned that he was also somewhat concerned because his driving habits fall into the "severe service" category of driving. He said he drives 420 miles per week with approximately 200 miles per week of "mixed" driving. He usually takes several very short trips per day of under three miles. In addition, he occasionally does towing, and on top of all that lives on a dirt road.

Since the sensor had no way of knowing what kind of oil he had in the engine, synthetic or conventional oil, it seemed a mystery to him why the indicator did not recognize or adjust for his severe driving. Furthermore, why the disparity between this Tahoe and last year's Tahoe?

INTERNET SIGHTINGS
A DejaNews word search on "oil monitor GM algorithm" yields a small number of search results. At the alt.autos.gm newsgroup, a writer notes that the "Oil Life System compensates for different driving styles and conditions.

The math model involves computerized monitoring of engine revolutions, operating temperature, coolant temperature, oil temperature, and other factors to calculate the rate of engine oil degradation and predict when it should be changed." It is an alternative to "fixed oil change schedules that may not be suitable for all situations."

Another newsgroup writer commented that the gauge is used more as a flag for people who use their cars in severe service conditions, noting that it will automatically come on "at the maximum 7500 miles which is the GM recommended interval for normal use."

In response, another newsgroup member wrote, "You're trusting GM on a maintenance item? I trust GM as far as I can throw my Cutlass. I'll bet anything the oil change gauge recommends an interval slightly longer than that which would maximize engine durability. Remember, these are the same folks who claim 100K between tune-ups."

This posting reveals a naked distrust of GM, perhaps simply because they are part of "Corporate America". For most Baby Boomers, the notion of "planned obsolescence" is likewise rattling around in our collective memories somewhere. My opinion is that we should not automatically assume malevolent motives to businesses just because they are big.

THE INSIDE STORY
As luck would have it, on my last day at the Specialty Equipment Manufacturers Association (SEMA) show in November, I was able to locate a representative from General Motors who could assist in clarifying some of these issues for us. For nearly half an hour I spoke with Al Cline, who works in the GM Powertrain, High Performance Vehicle Operations division. My speculation is that Mr. Cline knew more about these sensors than nearly anyone at the show.

By the end of the conversation he said I should fly to Detroit and talk with the woman who developed the GM oil sensor logarithm, Dr. Shirley Schwartz.

Here are highlights from my discussion with Mr. Cline.

1. The first cars with the oil sensor were the pushrod Cadillacs and Buicks in 1986, followed by the Alanti in 1987. As noted above, the sensor does not actually assess oil quality. Instead, it uses an algorithm to model oil life.

2. Mr. Cline does not believe in oil change intervals without monitoring the driving style. He said that some people, based on the way they drive, should change oil every 2,000 miles, others 12,000. There are too many variables, he said, therefore the entire industry is skewed because there is no such thing as a routine oil change interval.

3. GM believes the algorithm monitor is the most accurate way to measure when oil should be changed. It is adjusted for each engine because engines operate differently.

SUMMING UP
One thing is clear from this discussion: General Motors has a different philosophy about oil change intervals than the oil industry. While oil companies rally round the 3,000-mile-oil-change-flag, the auto industry is marching to the beat of a different drum altogether.

There's no doubt that motorists have begun relying on their oil sensor light rather than traditional oil change intervals. It's unlikely that we'll suddenly see an increase in the number of motorists who will read their owner's manuals all the way through. Going forward, we're more apt to see a continuing decline in the number of times per year people come in for an oil change, even if they live on dusty roads.

When cars are routinely going beyond 5,000 miles between oil changes, they need the extra protection that a premium synthetic motor oil offers. If customers don't want to get educated enough to protect their expensive cars by thinking through their service requirements, we can take the initiative and make the recommendation for them. The upgrade to synthetic is a small price to pay for the long term security.