I use 10W30 Chevron Supreme Synthetic (b/c there is nothing more to choose in 5W30 or 10W30, besides Shell)... Where I live, we never get even close to -18°C (0°F)... so using 5W30 is pointless to me... In the other hand, we are getting temps over 35°C (95°F)... I wonder if it be reasonable...