[m6pwr]

If you do have used oil analyses done, you might also consider Polaris labs http://www.polarislabs.com/, or Oil Analyzers Inc. http://www.oaitesting.com/index.htm. Polaris is ISO accredited. Oil Analyzers is cheaper and they actually are a "private label" for Polaris - - Polaris does the actual lab work. Blackstone is a well known lab but the only problem is they do not actually test or measure fuel dilution (an important metric in turbocharged direct injection engines - - like the n55 in the 535). Fuel dilution is a side effect in all di engines and aggravated by turbocharging. Blackstone uses a formula to estimate fuel dilution based on the degradation in flash point. In theory it should give an accurate estimate, but in actual practice it doesn't. I had a n54 with Dinan stage II and submitted samples from the same drain to both Blackstone and Polaris. Blackstone reported fuel dilution as nil (less than 0.5%); Polaris reported fuel dilution as slightly over 2.0% (the upper limit according to most guidelines), and this was with a very short oil change interval - - about 3k IIRC.

If I had an engine with an aftermarket mod that increased turbo boost, I'd keep a close eye on fuel dilution. Among other possibly bad effects, fuel dilution messes with the oil condition sensor BMW uses - - it keeps the dielectric constant artificially high and fools the ecu/oil condition sensor into thinking the oil condition is perfecto.

Hope this is helpful and not too pedantic. Oil is my favorite subject.