At the minimum you should be using oils that meet/exceed the MB spec for your vehicle... A C280 probably requires an SJ oil, I don't know if 100% synthetic was required then. As to weight, if it is cold you are worried about you can usually go with a 5W-x oil vs. a 10W-x oil, making sure you cover the higher number as required by the spec for your vehicle. MB's used to require both a gas and a diesel spec (i.e. SG & CF) for the gas engines. I just use 15W-40 Chevron Delo or Pennzoil LongLife.

That being said, the manufacturers almost always will agree that if you mix one weight of their SJ (or SL or SM) oils with another weight of their SJ oil there is no problem. Actually, almost any synthetic oil you buy will state it is compatible with any oil meeting the same API grade.

What you don't get is a new viscosity. 4 quarts of 10W-30 and 4 quarts of 20W-50 do not equal (or magically become) 8 quarts of 15W-40. You have 8 quarts of oil, 1/2 of which are 10W-30 molecules and the other half are 20W-50 molecules. This is not necessarilly a bad oil mix. Mixing brands adds the possibility of mixing different additive packages, which probably won't hurt your engine but may not result in the "ultimate" best mileage, performance, oil life, etc.

Just not really necessary. All the modern oils are really good, most are way better & some are totally outstanding. Delvac 15W-40 or 5W-40, Amsoil 10W-40 and 15W-40 would all probably work well for you. And there is an Esso 100% synthetic product that the Canadians on Bobistheoilguy.com just swear by!

Cheers!

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'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435