Again, you have to look at the pour values if you want a real look at the cold operating temps. Not all 20w-50's are the same, not even close. If Butler recommended that viscosity for your engine, then go through the list and find your favorite brand that has a pour value that works in the climate you live in.
There are a few good 20w-50's that will work in extremely cold temps, and some of the 20w-50 brands are better in the cold than some 10w's in other brands because the additive packages are simply better with some brands. So it pays to look at the pour values.
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