The dealer did 15W40 Mobil 1 oil changes.

Should I continue with 15W40 or use 0W40?

Why would the dealer use 15W40?? I thought 0W40 was the only approved oil for W220.

Some dealers switched to a special Mobil 1 5W40, although some have switched back for everything but diesels. The 5W40 is required for new diesels with advanced emission control systems. Chances are that's what they used. 15W40 would be a major problem. MB says you can mix the 5W40 and 0W40 in any combination. You might find the 5W40 hard to find in quarts.

what adverse effect would a heavier viscosity oil have on an engine as long as it's suited to the climate conditions that the car is predominantly used in?.i was a loyal mobil 1 0/40 client on my last E-320,at 66k miles i started hearing alot of clatter on first start of the day,lifters bleeding down,chain tensioner,lower end rattle.as soon as the oil pressure came up it went away.the 0/40 is the european formula?.i live in the high deserts and dont have the extreem cold that the "0" is designed for.im now using castrol syntec 10/40 on our S-500 and feel that it is better suited to desert heat.the only difference is a slight decrease in fuel efficiency,0/40 flows like water during warm up and is more fuel efficient.

It's more than the viscosity. The engine calls for MB spec 229.5 oil, and there aren't very many of those. The number before the W is the cold viscosity, so 15W40 is very much thicker when the engine is cold than the 0W40 the engineers called for. This is likely to cause increased startup wear and is not a good thing for your turbos. Why would you chance it when MB clearly states it voids the warranty?

im still sticking with the 10/40 castrol syntec because it does meet the MB spec and is listed as an approved choice for engine lubricant in the owners manual.i guess with turbos you want the oil there as quickly as possible.iv'e seen turbo bearings and they are nothing more than ported bushings,there is literally no mechanical contact between surfaces as long as the oil pressure/flow is never interupted.so you would still run 0/40 in all conditions,even desert heat that ranges from a low of 70 and a high of 118F?.I respect experience and would not want to make a mistake based on my old school mentality.thanks.