I believe that regular oil (non synthetic) should be changed every 6 months or 3,000 miles, which ever comes first. While you can debate the exact intervals, the time limit is based upon the tendancy of oil to build up acids that can damage internal bearings. For a car that has fresh oil and is never driven I'd expect you could go longer than 6 months. If, as you have, you run it once in a while (which it should be to keep your seals pliable) the acids will potentially build up and you don't want to leave it in there too long. As to the filter, I'd guess the old filter is still fine but be sure you drain all the oil out of it (pretty much automatic on most Mercedes). One important detail, if you only run the car a little, be sure you get the engine (and more importantly the oil) up to full operating temperature so any condensation in the crankcase has a chance to boil off. Water in the oil, even if only a little, can do as much or more damage than tired oil.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold