Just an obvseration:

If you've always used 15W40 and you just started to have problems running rough/starting I REALLY doubt it has anything to do with the oil your using. Personally, I use Mobile 1 0W40 in my 94 E320, but I know that thats not what has always been used (I purchased the car a year ago). I think for a colder climate (like B.C. in your info) a lighter weight would be a better choice (5w40, 0w40, 5w30 etc.) but its your car and your wallet.

Good luck!

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE