My new to me 2003 Ford Windstar recommends using 5W20 and no other weight of oil . I know this is due to CAFE and such but 5W20 can't meet all types of driving conditions or can it? We are heading across the country this summer through the southernm states, read extremely hot and mountainous terrain. Can a 5W20 handle the heat? I am going to go synthetic for sure but what weight would be good? I have read that the 5W30's sheer too much, would 10W30 be the way to go?