I use Mobil One 10w-30 all year round here in South Florida in my 850 Turbo, thats all I use too. As well as the Mobil One M1-204 filter, its a little longer than the M1-104 and easier to grab and offers a little bit more filter area. If you're getting good oil pressure with the 10w-30 thats what I'd stick with then. Too thick of a viscosity will trigger filter bypass at mid to high rpms and thats where you drive most often. The 5w or 10w-30 weight oils provide good flow characteristics and plenty of protection without hitting the filter's bypass (flow resistance) pressure.

Mike