I prescribe to the Larry Bible school of oil changes. I changed the oil after my 50 mile trip from work yesterday. I also let the oil drain for a good 5 hours. Due to the fact that I put on only 5000 miles/year, I change the oil twice per year. Once in the fall (winter oil OW-40 & install snow tires) and once in the spring (summer oil 15W-50 & remove snow tires).
I do all of my maintenance so that I can inspect my cars for any potential problems. Yesterday I found a second dead mouse in my E320 stuck under a rubber flap next to the head light assembly. The first one I found two years ago was traped in a frame rail. That car has proven to be a good mouse trap.
You can mail order Mobil 1 0W-40 from http://shop.store.yahoo.com/parkplacemotorcars/mob1trismoto.html.
It will send you back $5.37/qt plus shipping. Another approach is to look up fuel oils in you local phone book and let your fingers do the walking.
I found it unusual that my local MB dealer (parts dept.) doesn't carry Mobil 1 0W-40.