Kilgore, you posted "My SA would benefit from me betting more frequent oil changes. Yet, he discourages me from doing so because based on his expertise and experience, he does not view it as necessary." Do you really think that a SA is going to go against BMW Munich and say that you should get an oil change more often than it is required by the warranty? Do you see any opportunity for a lawyer to take any such suggestion and use it against BMW in a lawsuit over a sludged engine with 15K intervals?
I disagree with Mike Miller on a few things, but not on oil change intervals. Modern oils are far better than what was available only ten years ago. They can keep their lubricity, their viscosity, and their detergency for 15K miles. In the lab, but not in every engine in the real world.
Salesmen who get onto highways in Nevada and drive all day without a complete cool down have the best shot at getting 15K safe miles between changes. A commuter in a congested New England city isn't going to make it. His oil never gets hot enough to boil off the condensation collecting in the sump. Ever see water almost pouring out of a tailpipe on a Winter morning? It's not all going out the tailpipe. Shear forces ('scraping' of oil with suspended water droplets) is going to mix some of that moisture with the oil. In the gasoline are sulphur molecules. These molecules mix with water molecules to form sulphuric acid. There are buffers in oil to fight acids. They run out. The moisture in the oil reduces the vaporization point of some of the oil. It boils off when the engine is shut down. It then passes through the system and condenses on the fastest cooling metal, forming deposits which eventually turn into sludge.
Engines are started at 10F and then run at 200F and shut down. That kind of thermal cycling presents challenges to viscosity integrity you don't get in southern California. Where will oil last the longest? BMW always blames our sh*t gas for all kinds of mechanical and driveability problems. Do you think that maybe there are also things in the gas which result in risks to the oil? How about methanol blends, or base stock composition, or refinerey practices, or additive qualities? Your intuition has to recognize variability in all those areas. To think that BMW has evaluated all the potential hazards to oil reliability is absurd. If so, then one would have to say that to ensure every BMW engine will perform at peak reliability for as long as possible would require that oil last 40,000 miles in the very best circumstances. Because that would be the top end of the failure curve, and 15K would be the lower end. How likely is that?
Last edited by DSXMachina; 12-07-2012 at 11:51 PM.