"light enough" depends on how much you are trying to pick up!!
But just to confirm, what kind of engine(s) and single digit Fahrenheit, right? When in doubt, I defer to the directions. Rest assured, the Germans did NOT put any guesswork into things like that. I use 5-40 synthetic for the the benefit of quieter lifters, the slightly extended drain interval and easier starting (3 bad glow plugs and started at 35 degrees) are a good side effect.