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

Thanks for the response. 10W40 Is something I am considering. About the climate, I'm in the Vancouver area, our weather is like Seattle's, and we don't get any winter snowfall or winter freezing. Alot warmer then Minnesota. I've just been told by many people to avoid 15W40 during winter freezing, so its really left me wondering if its my oil getting slushy...or what climate I should drift away from 15W40 in, or all together abandon. Also if their's any benefits to switching to a synthetic blend? Thanks

I have a 300SD 1979 I use Mobil 1 extended performance 5/50w. Great product and you can get it at Walmart