I've been using Valvoline Maxlife's 10w-40 in the winter, and their 20w-50 in anything wrong with that? I've heard a lot of people swear by the Rotella 15w-40, thinking about trying it.

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C