Today Mercedes-Benz recommends 0w-40 for ALL vehicles and they explicitly state that the current chart (which also lists 0w-30, 5w-40) supercedes ALL other viscosity charts. There is NO good reason to run 20w-50. Your owners manual was printed a LONG time ago when Castrol GTX had a Cx rating along with the Sx rating. That changed in the early 90's when it became difficult to meet the increasingly stringent Sx ratings as well as the Cx ratings - for a MINERAL oil. Today's off the shefl synthetics still meet the CF and Sx ratings - such as Valvoline SynPower 20w-50 or Castrol Syntec 20w-50. IF you don't want to go the synthetic route, Chevron Delo 15w-40, Mobil Delvac 1300S 15w-40 are two of the best choices.

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Brian Toscano