I switched to Mobil 1 0W-40 two years ago, mainly due to the cold Chicago winters. Since I switched to 0W-40 I havn't had the "white sludge" build-up under the oil fill cap which occured in freezing weather on short trips which didn't bring the engine up to temperature.

I use to switch to Mobil 1 15W-50 during the summer months but this year I decided to stick with 0W-40. 0W-40 is rated for year round use with the "newer" (112) Mercedes engines because of the tight tollerences.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles