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Old 01-25-2005, 01:24 PM
HughH HughH is offline
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Location: Arlington TX
Join Date: Jan 2005
Posts: 78
Mein Auto: 2004 325i
Attached is the discussed segment of the February edition of Road&Track:

This is a great question we cannot answer definitively on a mechanical basis. It’s not from a lack of research, but rather the limited data available and the large number of variables involved simply won’t allow putting this question to rest.

If you’ll let us put forth our hunch, however, our guess is the extra oil change will not yield extended engine life. Assuming a proper oil level, operating conditions play a greater role in engine longevity compared with what we’ll assume is a minor amount of dirt or a somewhat depleted additive package in the oil.

Let’s consider your well-driven, well-cared-for BMW. Because synthetic oil is so stable at high temperatures, it does not so easily cook into uselessness like mineral oil, which is where the extended oil change intervals mainly come from. Assuming a serviceable, un-punctured, not over-loaded air filter, tight engine sealing and paved roads — some of us still motor over dirt driveways or roads — then the oil filter ought to trap what harmful particles come its way for 15,000 miles. Given that, typical use should not allow much wear of the bearings or cylinder wall environment (piston and rings) or the valves and their guides.

Allow dirt into the system via cracked seals or an overloaded air filter, and engine wear will accelerate because of dirty oil. Then, of course, frequent oil changes help.

But the real disaster is no oil, and not just from low oil level on the dipstick. Winging the engine to redline and holding it there, especially on a cold engine, might pump most of the oil into the valve cover before it can drain back enough to keep up with the oil pump’s demands. Then you have real engine wear. Hard, sustained cornering can force oil away from the oil pump pickup in the pan, accomplishing the same thing.

Furthermore, consider that most engines go out of service for reasons unrelated to oiling. Very often the rest of the car wears out first; cracked dashboard padding, leaking weatherstripping, faded paint, crash damage or even plain old boredom on the owner’s part has sent more modern engines down the road than wiped-out bearings or scuffed pistons. And if the engine does fail, often it is the highly stressed valvetrain that gives up first.

Our guess is that sticking to BMW’s official oil change recommendation and scrupulously maintaining all other aspects of the engine will return maximum engine life. As for your Honda experience, those engines use very fine screens on their oil pump pickups, along with mineral oil. Gunked-up oil doesn’t flow well through those tightly meshed screens, so extra diligence in the oil change department pays off with those engines.
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