'99 ML320 purchased in '98. Castrol mineral oil and filter every 5,000 miles. Gutted the cats immediately and turned off idiot light with O2 simulator. Run the cheapest gas I can find as combustion chamber temps are down w/o the EPA exhaust potatoes and the computer doesn't sense knock and detune the engine for reduced gas mileage.
Replaced crankshaft position sensor, mass air sensor, and unclogged EGR pipe; nothing important. 227,000 miles. Am installing transmission fluid temp gauge as I need to do some towing.