Automakers have been recommending 5W30 oil in most new cars/trucks for several years now. And Ford and Honda have been recommending 5W-20. What is the truth about all of this? Is it really better to run 5W-30 throughout the year, including the summertime (or 5W-20 in the case of many vehicles), or is it better to run 10W-30 in the summer? I have never been convinced that 5W-30 and 5W-20 oils are the best for all year use. I think these weight of oils are selected mostly to meet CAFE requirments. In seems to me that 5W-30 and 5W-20 (and 0W-30) would be best foe the winter months. I once came across some information that many of the very engineers (speaking for the automakers) who were recommending 5W-30 oil throughout the year were actually using 10W-30 in their own cars/trucks in the summertime. And in Europe, people are often using thicker oils, sometimes in vehicles that have the exact same engine as vehicles sold in the US. You will hear about oils like 0W-40 and 5W-40 being used in Europe, but in my car's manual it says that the only viscosities that can be used are 0W-30, 5W-30, and 10W-30, with 5W-30 being preferred. It says that 10W-30 should be used only in warmer temperatures and if 5W-30 is not available. 0W-30 is acceptable in the wintertime.