20W50 is totally fine for the spring/summer/fall months. It’s actually rated to flow down to -4F, but I personally would not want to start it below 32F just to be extra ‘safe’. As far as the Viper goes, if it were mine, it’d have VR1 10W30 in it, no question, or Brad Penn 10W30 if you’re like me (which I know your nasty ass is) and get that Christmas feeling when something comes in the mail lol.
EDIT-I’d use either the non-synthetic or synthetic, depending on drain intervals. (non synthetic for more frequent intervals).