Depends on the climate really... but mostly you want lower weights for colder weather. I use 5W-30 or 10W-30 in the summer months and 0W-30 in the winter months. I initially thought that since both oils flow at temparatures as low as -57 F, the difference wouldn't be much. Then one day when it was -30 out, I left one quart of each outside overnight and the next morning, the 0W poured out a lot quicker than the 5W did. I switched to 0W.