I'm not an expert, but it seems that most modern brand name oils today would be good past 4k miles in a modern and otherwise well maintained engine. I'd even venture to say that 9-10k miles on regualr dino juice is ok (but why would you at .99/qt). Synthetic is even better. IMHO, the weak spot here is the filter. Have you compared a filter with 4k and a filter with 10k? I have not on my MB, however, I have on other vehicles. The 10k vehicles filter is by far more crapped up(technical term) and restricted than a 4k filter. Cutting a couple open will prove that to you.
I plan on at least changing my filter in 4-5k intervals. Seems relatively easy and cheap to do on my C320. I haven't yet decided about waiting or not for the fss (10K+) to do the oil itself.
God, I hate motor oil threads :-)