The calendar-year changes are mostly a thing of the past with modern oils. The OCI should be dependent on the specific oil you use and the type of drive cycles you operate on. Even then, the modern oils are much better at handling moisture and acids that can form when the oils doesn't get up to temperature for extended periods. When they reach their end of life, this ability can be reduced.

If your oil "sucks" then it might not support long OCIs well. If you accrue 11k miles over 20 years driving a mile to and from work every day, that's probably not conducive to longer OCIs either.

Really, the best way to determine your OCI by having the used oil analyzed to see when the TBN drops below an effective value, when wear metals increase above a desired threshold, the flashpoint is lowered, it thins out too much etc., etc.

Or don't. I'm not surprised you haven't had problems related to oil. Modern synthetics are fantastic and oil-related failures are ultimately rare no matter what oil is used. Also, 20k miles is not crazy-long. It's more than I'm comfortable with.

Oil is a contentious subject. It doesn't have to be. The oil I use in my diesel X5 is made by Mobil and although I do prefer Mobil 1 products, ultimately it's an oil that suits my diesel engine better than most with specifications that matter to the modern DI turbo-diesel: lowest SAPS, high HT/HS, stable 30W viscosity throughout life (based on my OCI analyses) and the lowest Phosphorus and Zinc. All specs that matter to a modern diesel with emission reduction systems.

What I'm trying to say is that with used oil analysis (UOA), review of the oil specifications, consideration of your vehicle and how you use it, it should be no problem determining what oil to use and how often to change it and be sure your maximizing your dollar and vehicle life.

That being said, you really don't know if dirty oil, or more aptly, your oil's inability to handle the increase in wear metals and the like, has contributed or is contributing to VANOS maladies that might be causing you issues. It is a fact that VANOS can be finicky and difficult. Fortunately, there is tons more VANOS information out there now than the dark days of the last and early this century. At 11k miles, oil is probably not your problem.

Ard is talking about the double VANOS on the E39 M5. It uses two VANOS banks that each control the intake and exhaust cams over each head. As an L6, you probably have a single bank controlling the intake and exhaust cams but some engines had VANOS that only controlled a single cam.

The VANOS test may reveal interesting data as would a UOA.

__________________
8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim