Remeber OE does not equal OEM.
OE parts generally have a higher Quality Control, but not always.
One thing OE Mercedes parts have over an aftermarket or OEM part is that Mercedes will standby their product not only for 1-year, but also for the labor, and most of the times, any incidental damages should that part fail.
Most dealer parts guys don't know this, but ask Stu Ritter (MBCA Tech Editor) and your local Mercedes Zone Rep, and it's true.
If I'm rebuilding an engine, or any system that is expensive to repair (parts or labor) or critical, I use OE parts. If I install or local independent shops installs an OE headgasket, and OE headbolts, should the engine fail because of the headgasket, Mercedes will 99.99% foot the bill to repair (parts AND LABOR) and/or get me a new engine.
In the case of my independent shop, this doubly-protects them. That's why when Stu ran his shop, he used only OE parts on those expensive/risky jobs, esp. if your indy pays their techs well.
In some cases, like waterpumps, only the OE pumps have the correct vanes to provide low-speed circulation. Weatherstripping and door seal that are OE also have more rubber content (meaning more expensive, but more durable and better sealing) than aftermarket, which has a higher carbon-content (deteriorates quicker), or OEM which don't always fit.
1988 360TE AMG