whatever engine manufacturer says to use it,
e.g. mercedes benz and BMW say to use their own spec 5w40 year round. Benz and BMW are built a bit looser (dheela) - heavier oil works here.
Ford, GM, toyota, Honda etc. are built quite tight now and to maximise fuel efficiency use the lightest oil possible. No problem with that as long as the sump capacity and oil temperature is kept under control.