I joined the Mercedes-Benz Club of America and enjoy a 20 to 30% discount on parts and labor at the local dealer. This makes them very competitive on most parts, so I buy things like ignition parts and gaskets and filters from them. That way I get the OEM stuff and do not worry about it.
I use the recommended lubricant ratings but use only synthetic oils. I have done this since I noticed Mobil 1 on the shelves in the mid 1970's. I use the recommended 0W-40 Mobil 1 in the engine on my gas cars, and Mobil Delvac 1 5W-40 for my Diesels in Winter and 15W-50 Mobil 1 in the summer.
I use Redline transmission lubes for the manual transmissions (have not changed the auto fluid yet as the job was done when I got the car a few months ago, by the dealer) and Redline high shock gear oil in the differentials.
I use MB radiator fluid from the dealer, as well as the MB fluid for self leveling system as I could not find a synthetic version anywhere by Mobil or Redline that I was sure was the right stuff.
Well, that is one solution for buying parts you have confidence in and I am sure many others have their own sources and methods that also work. Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)