7 1/4" Lubrication

So what gear lube should I use when its negative 40 outside. Im serious I live in Fairbanks Alaska and drive a 66 v200 valiant slant six that has seen better days and warmer weather if the sand that was under the carpet is an indicator. It is an amazing car that I bought as a parts car for my 65 Dart GT project and ended up driving it.