Which Viscosity of Oil?

Assuming the minimum outside temperature is 50ºF, then the chart recommends that any viscosity oil can be used - anything from 0W-30 up to 20W-50. This seems like a pretty wide range of viscosity weights. Even temperate West Coast temperatures would allow for a pretty wide range of viscosity choices. Are there any 'rules of thumb' in further determining which is the best viscosity for any given situation?

Curiously, the Owner's Manual for my '96 SL600 does not specify a viscosity; only the quantity. The only reference in the entire manual is on page 146, where it simply says "Recommended engine oils" beside the quantity needed

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alexlennox	Which Viscosity of Oil? Assuming the minimum outside temperature is 50ºF, then the chart recommends that any viscosity oil can be used - anything from 0W-30 up to 20W-50. This seems like a pretty wide range of viscosity weights. Even temperate West Coast temperatures would allow for a pretty wide range of viscosity choices. Are there any 'rules of thumb' in further determining which is the best viscosity for any given situation? Curiously, the Owner's Manual for my '96 SL600 does not specify a viscosity; only the quantity. The only reference in the entire manual is on page 146, where it simply says "Recommended engine oils" beside the quantity needed