Dave,
Welcome to TBN. The operator's manual will provide the very best advise on viscosities at those temps. And I would recommend any major brand that is diesel rated. Thank the Lord - you didn't ask synthetic or regular. That could generate a never ending diatribe.
Actually the best advise at -40c (same as -40F) is a cheery fire in the hearth and a yet to be opened bottle of Jack Daniels. Man, if you are out at those temps - you will probably be the only moving thing in the local vicinity. I've operated equipment at those temps - Glennallen, AK - and things can have a habit of going wrong.