I have a Ford Focus and use 5W20. Though I intend to keep using 5W20 would it hurt the car if I used 10W30?
First number tells how good your car starts when it’s cold. If you live in Miami, you don’t care. If you live in upper New York, you want it to be 0W for winters.
Second number tells how liquid you oil is when it’s hot and you want to go with what your owners manual says, period. Unless you want to ruin your engine
What does the owner’s manual say to use? As long as you are within the weight listed there, you are good.
Ford’s way to meet CAFE requirements is to specify the thinnest oil possible that gets the car through the waranty period. As mentioned in a previous post, if you put cheap 5W20 in and drive through death valley in the summer your engine will be very short lived, but Ford knows that these instances are few. As the post says, the 10 part of the Grade is for starting, and it’s OK in Miami, but not in the Northern states in the winter. I would use a synthetic 5W20 during the warranty period , and switch to 0W30 or 5W30 for winter and 10W30 for summer after that. This is a clear case where the long term interest of the user is thwarted by the quarterly dividend of the manufacturer. In other words, Ford does not care if your car lasts BEYOND the warranty period, as long as they get the most CAFE credits.