I was working at a bbq fundraiser with the head of service at a large Ford dealership. Asked him about 5W20 oil. He said he had formerly used 15W40 in his own cars, but that modern Ford engines had very tight tolerances and that 5W20 was the only preferred weight. Heavier weights could, theoretically damage these engines. I'm still up in the air on this entire subject and glad I drive a Hyundai that actually calls for 10W30 as preferred!