Royal Purple would be the best in my opinion. The film strength and anti-wear properties of Royal Purple at low temps (relative to an engine) are rather noteworthy.
Stick with a 0w or 5w oil with the lowest numbers (ie. 0w20, 5w20 and not 5w40, 15w50) as the viscosity of the thicker oils may prove problematic and don't offer any extra protection.
