10w30 synthetic is a good all-year application. The hotter the engine runs .. the thicker viscosity you want. Good to have low viscosity for start up (first number small). Lots of higher power cars would probably use 15w50/20w50/etc. The warmer the climate and the warmer the engine ... the more you should consider thicker viscosity. Lower temp ... less agressive driving .. more you consider lower viscosity. TMK.