Toyota V8 4.0L: 11 years old with 90,000 miles Toyota V6 3.0L: New with 2,000 miles The owner's manual for both states either 5W-30 or 10W-30, with 5W-30 "recommended". My assumption is that 5W-30 is recommended for all-weather protection. Where I live, the temperature is between 50F-90F year-round. Does this make 10W-30 the better choice? I'm pretty sure it does for the higher mileage engine. But am not sure for the new engine though. I've bene using 5w-30 in both cars. A couple of related questions: 1. Are VIs still a factor between 5W-30 and 10W-30 with SL rated oils? Or does it vary by manufacturer? The main reason to go with 10w-30 would be less VIs. But I read somewhere there's no longer much of a difference between 5w-30 and 10w-30. 2. I hear 5w-30 is more "fuel economic" than 10w-30. Would the difference in fuel economy ever be noticeable? 3. For a car that's not driven often (once every few days), is a thinner weight oil better?