Here's my opinion on this topic based upon what I've read here and my own research in the past.
Originally I asked this question because some engines, based upon design are carbon collectors and run dirtier more so than others and I'm not all that familiar with the BMW Inline-6 - especially the N54 as it pertains to design and engineering.
Case in point, I own a 1990 Corvette ZR-1 that I've had since 1994. Factory fill was non-synthetic 10W30 - and I switched to full synthetic. I did have Blackstone Labs run a couple periodic tests and they always came back normal showing no signs of abnormal engine wear and the oil was always clean after 5,000 miles.
The ZR-1's LT5 engine is known to build carbon more so than regular engines because of its design and how the PCV system was engineered. Hence the reason why, if you drive the car a lot in stop and go traffic or around town, it's always good to take it out on the highway, wind it up and exercise the engine.
The ZR-1 is my weekend toy. I only take it out in the summer during good weather and it sees nothing but highway miles and plenty of exercise. No matter at what intervals I've changed the oil, it usually always comes out looking pretty close to what it looked like going in.
The BMW is obviously my daily driver so it sees 1/2 highway miles and 1/2 around town miles per day. It also sees a lot of stop and go traffic to and from work - through highway construction projects that will be going on for at least 3-5 years to come - hence a lot of dust.
In my opinion, I think 5,000 mile oil changes is probably overkill....given that 3,000 mile oil changes was once the recommended requirement for non-synthetic oil based cars.
Given my driving conditions; higher levels of road dust from construction, traffic, etc., 15,000 miles is probably a little too high and 7,500 miles would probably be better. If I was beating the hell out of it every time I take it out and tracking it on weekends - I'd probably drop it down to 3,000-5,000 just to be on the safe side.
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