The 40 will be better in the summer, but the 10w might be a bit thick in
the winter if the car sits outside for any extended period (seen 10w
used where after a -40 night, it is like toffee) Saabs in most of the
world use 10w or 5w oil all year round even in turbos, although some use
0w, except in cold weather countries like it's native Sweden. In Winter
the Swedes use block heater over night, else when they start them up the
oil looks like fudge even with a 0w40 or 0w50 oil.

Have of extreme low temp case where oil filter have burst because the
oil is too congealed and is forced rather than pumped through the filter
like Playdoh.