I talked to MBUSA about this when we bought our E430. They said:
1) MB has determined that there is no longevity advantage to changing the oil at intervals shorter than the FSS calls for.
2) Make yourself happy and do what makes you feel good (other than ignoring the service recommendations), but it isn't called for and will not violate your warranty.
3) M/B does not require or recommend the use of synthetic oil in cars not filled with it at the factory, but make yourself happy and do what you like as long as you follow the List of Recommended Products, it will not void your warranty.
As an aside, M/B is responding to the complaints about high maintenance costs (as distinct from high repair costs). If you ever owned and followed the service ritual for a 113,108/109, 110/111 or earlier car, the preventative maintenance items were MANY and labor intensive. If followed, the cars were quite reliable, but the idea was to identify problems in the shop rather than on the road, and often, problems were uncovered during a routine service. The short service intervals did not necessarily prevent problems, they just were identified early. M/B has gradually reduced the preventative maintenance items and extended the intervals. I always used to follow the recommended intervals (3000 mile oil/filter and chassis lube, etc)with my old cars. When I bought a 300TE I cut the service interval (7500 miles) in half. I had the worst performance of items that I thought I was protecting (valve guides, camshaft, etc) of any M/B I've owned. I'm not saying I caused the problems; I simply did not prevent them. On my 240D, which I bought used, the original owner followed M/B recommendations to the letter, but did not exceed them. The car is approaching 200K and shows no ill effect from the 5000 mile oil changes, etc. So what am I doing on the E430? Every 5000 miles I change the oil and filter. I just can't get myself to wait for the FSS. Will it last longer? I don't know, but I'm happy. Sorry for the long post.