Depends. Since I am a 'real expert' in this area (for real)-a few words. (For gasoline engine drivers) Personally I run Mobil 1 in my 560SEL and change it twice a year (about 5000 miles) Do not try and run any oil longet than that unless you live on the freeway and rack up lots of miles. Freeway cruising use would be good for up to 15,000 miles with Mobil 1 and similar products.
For almost all town driving, the best and most economical system is premium quality 10W-30 or 10W-40 changed (with filter) every 3 months/3,000 miles. Oil is real cheap compared to hard parts. But buying good gasoline is equally important. Stick with major branded gasoline. Here in Houston, Chevron and Texaco are the two best. Fuel detergent additive package is vital to engine cleanliness and life.
For MBZ diesels, use products recomended in your owners manual with faithful regular oil and filter changes (3000 mi max), buy good fuel, and keep the injectors clean and working right.