this past summer I ran 10w-40 in my Zetec. I figured it couldn't hurt becasue of the higher temps of the summer and the miles (105k at the time) on the car. I have 5w-30 back in it because there wasn't enough 10w-40 for me to change the oil when I did. The only difference I notice was I think my temp guage was just a tad bit lower then before.
