My vote would be Amsoil 4-Stroke 0w40 because generators sit for months without running. This oil states,
"It is anti-rust fortified for superior rust prevention, which is especially important in seasonally or infrequently used equipment."
And
"its broad viscosity range and wax-free formulation make it excellent for use in both hot and cold temperature extremes. As an SAE 40-weight motor oil, AMSOIL Formula 4-Stroke Power Sports Oil provides outstanding protection for hot-operating engines, while its 0W rating and -60°F pour point ensure easy cranking, excellent cold-weather starting and quick post-startup protection."
"It is anti-rust fortified for superior rust prevention, which is especially important in seasonally or infrequently used equipment."
And
"its broad viscosity range and wax-free formulation make it excellent for use in both hot and cold temperature extremes. As an SAE 40-weight motor oil, AMSOIL Formula 4-Stroke Power Sports Oil provides outstanding protection for hot-operating engines, while its 0W rating and -60°F pour point ensure easy cranking, excellent cold-weather starting and quick post-startup protection."