(AOL Autos) -- I have been in the auto repair industry since 1972. Hmmmm, a simple calculation of the math illustrates that I have been in car repair for, uh, well, let's not go there. It makes me depressed. You do the math and keep it to yourself. At any rate, enough about my mid life crisis. Let's talk about oil change intervals.

Oil changes can be done every 4,000 to 5,000 miles instead of the previously suggested 3,000 miles.

For years the accepted oil change interval (as per the carmakers) has been every three months or 3,000 miles, whichever comes first. Why? Because the oils of yesterday degraded and broke down when left in the crankcase environment for longer than the prescribed interval.

The combination of heat, friction, and the oil oxidizing over time resulted in an unholy clothing of the engine's internal parts called sludge.

As an automotive machinist for a good part of my career, I can tell you that sludge is an engine killer. Sludge takes a greasy, cake-like oily form and plugs oil return passages and acts like a sponge and soaks up good oil to grow its grotesque form starving the engine of vital lubricants. Once established, engine heat crystallizes it to a hardened rock of ughhhhhh, I have spent many an hour scraping and yes, sometimes chiseling established sludge from the inside of an engine before performing a machining operation on it!

As the machinist prepares to perform a machine operation on a cylinder head, crankshaft, engine block or the likes he/she must clean their work meticulously before performing the prescribed operation. If the sludge is not cleaned properly, the result will be a failed engine.

Why this lesson about sludge? Because without clean good quality oil in your car's engine, it will develop sludge and cause premature engine failure.

Now more than ever before, vehicle engines operate at high heat and close tolerances. The reason for this is the Federal Emissions mandates the government has imposed on the carmakers.

Vehicles have to emit a smidgeon of the tailpipe emissions they did a decade ago. Carmakers have risen to this occasion by increasing combustion chamber temps with higher compression engines, running leaner fuel systems, adjusting ignition timing for optimum emissions, narrowing cooling system water jackets, and tightening engine oil tolerances.

All this makes for hotter running engines that emit less tailpipe emissions. Putting these demands on engines requires a lubricant that can stand up to this harsh environment.

Hence, petroleum companies' work hand in hand with carmakers to develop engine lubricants to meet the requirements and demands of a particular powerplant, still delivering the advertised horsepower and torque output that consumers expect. Research and development between carmakers and petroleum companies has resulted in improved engine lubricants that properly lubricate your vehicle's engine as well as keep the inside clean of sludge buildup, and can go longer between oil changes!

A pioneer in extending oil change intervals ...

A few years back, GM introduced a system called the OLM (Oil Life Monitor) system. This system had been in testing since 1984 and actually was put into some Buicks on a test basis. The goal of this system? To extend oil change intervals and attain bragging rights to having a more maintenance-free vehicle. The OLM monitors crankcase temp, moisture, and combustion chamber events (this represents the actual work the engine is doing while in operation).

By closely monitoring these elements of engine operation the system can measure the serviceable life of the oil to within 10 percent. After officially introducing the OLM with virtually no engine failures attributed it the OLM, GM changes their service recommendations to what they called an "Enhanced Maintenance Schedule." With the Enhanced Schedule, the motorist need only follow the dictates of the OLM and have other scheduled services done at prescribed intervals.

Ford Motor Company has followed GM into the extended oil change interval march. Four months ago (March of this year) Ford announced that they are revising engine oil change intervals to every 7,500 miles.

Ford told The Associated Press last year it was increasing the recommended time between oil changes because extensive testing indicated oils now were better and improved engines operate at higher a temperature which helps burn off the gunk in oil. The automaker also cited the financial and environmental benefits of the adjusted standards.

Yes oil is much better than it used to be, engines are better protected with today's new lubricants but the same old logic still applies to the oil filter: always use a good quality filter when having the engine oil changed.

The filter is the storehouse for dirt in the engine and when it doesn't do its job, the engine suffers internally. When dirt and grit are allowed to circulate over, within and on the engine bearing surfaces, cylinder walls, crankshafts, piston rings, camshafts and virtually all metal mating surfaces, they are damaged resulting in wider oil tolerances, lowered oil pressure and ultimately premature engine failure.

While I concur with R&D results over the years with respect to oil change intervals, I am still squeamish about leaving petroleum-based oil in an engine for 7,500 miles. I guess I just know too much based on personal experience.

So while yes, I have revised my thinking regarding extending oil change intervals:

Here are my revised oil change recommendations: change regular petroleum based oils every 4,000 to 5,000 miles and synthetic every 5,000 to 7,000 miles.

There, those of you that for years have criticized me as being in the back pocket of petroleum companies for recommending 3,000 mile oil changes, are you happy now? E-mail to a friend

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Tom Torbjornsen is a veteran of 37 years in the auto service industry, an automotive journalist registered with IMPA.