What are you guys, all over the age of 50?? Oils have come so far recently its amazing. First of all, if you're so stuck on the old methods of doing things(no offense) enough to still be using dino oil, dont bother reading further, continue to throw away your money and feel safe about your engine.
If you want to do whats absolutely best for your benz, like most of the ppl on these forums, you'll use a full(not a blend or fake synthetic like castrol syntec) synthetic. These oils protect better, and dont make the sludge/deposits that conventional petroleum based oils make. Dino oil will form sludge and dirt -and reduce your engine's performance over time.
Secondly, you guys are just guessing at your oil drain intervals. This can only be correctly/safely acertained through oil analysis. The guys at Oil Analyzers can help you out, as well as other labs. The test is about $14 dollars and will give you a breakdown of the oil, on many different levels - (chemical content, tbn, soot %(key))...
Point in case, if you use the best oils made for a diesel engine - Mobil's Delvac 1, or Amsoil series 3000 Synthetic Heavy Duty Diesel Oil, you are able to use longer drain intervals(anywhere between 5-15k changes), while still remaining CLEANER and more protected than your dino oil - at 3k changes. This is achieved by better esters, soot suspension properties, and additive packages that break down less. The cost difference between conventional and premium synthetics is made up in the longer drain intervals and the superior protection/cleanliness.
If you are really looking to keep the oil clean and extend the life/durability of your car, look into installing a bypass oil filtration system - available through a few companies - amsoil(
www.amsoil.com), gulf coast, oilguard, and comi industries are a few. Amsoil makes ready to install kits, but usually for more popular vehicles. Unfortunately I can't find one for my car, and am looking into an installation now, just delayed as I haven't found an oil sending unit adapter for the line to the bypass filter.
In short (this was a novel) use quality synthetic oil, and go for a longer interval than your dino oils. Your engine will be cleaner and last longer.