Well, early W210's with the 3.2l V6 suffered from sludge buildup when owners mistakenly used non-synthetic oils, paper media filters and then tried to use the FSS to determine oil service cycles. Although rare a heavily sludged engine is pretty much toast unless you are able to tear in down entirely and clean it. Cost at a dealer would be more than a replacement.

So if you want to use a non-recommend oil, I'd say shorten your service cycle to 4-5K. Less for heavy service like dusty conditions or mostly stop and go. The irony is that if you do the math, 3 oil changes and paper filters costs almost the same a 1 synthetic oil change and a fleece filter.

As for availability, Mobil 1 European Car Formula 0W-40 (229.5) is available from amazon ($7/qt) but may also be available at PepBoys, Kragen (in the west), O'Reilly's, Autozone, etc. Okay you get the idea.

Also, the doc lkcris posted lists the 229.3 oils which are also recommended for any of the gas engines (except AMG) prior to 2005.

Good luck whatever you decide.