When I spend winters in northern Maine, I use Castrol 5w-50 in my
wasserboxers.  When the engines are cold, they "Think" the oil is 5
weight ... allowing them to start in subzero weather.  After the engines
warm up, they "Think" the oil is 50 weight which is what a running
wasserboxer seems to like.  BTW, the "W" in 5w-50 stands for "Winter".

Mike
Houlton, Maine
Glade Spring, Virginia