Why would it be better to use oil of lower viscosity to lubricate engines in vehicles used in polar expedition?

It is not a matter of lower viscosity, it is a matter of the correct viscosity at a much lower environmental temperature. All oils are less viscous when warmed up, even the so-called multigrade oils, so the oil which would be appropriate at sub-zero temperatures would be relatively thin, and wholly inappropriate, in a normal climate.Why would it be better to use oil of lower viscosity to lubricate engines in vehicles used in polar expedition?
Oil becomes more viscous at lower temperatures. Higher-viscosity oil makes a ';freezing-up'; less likely.