10W40 Mobil 4-T is A-OK for year round use. That's all I ever use, even in 118 degree summer temps. Never had a problem; close to 40,000 miles. Tried some 20W50 one summer, my mileage dropped by about 5 mpg. Went back to 10W40 and stayed with it.
Contra todo mal, mezcal; contra todo bien, también
