mrlawnguy is accurate in his response that the Ford would have to test the oil, but even then, oil grades have a range. There are some 5w-30's that start out on the thin side of a xW-30 and can end up in xW-20 range with use. For the most part, the bigger concern is whether or not it was changed recently and isn't outside of the recommended intervalCorrect me if I am wrong, but assume the time the car was under warranty. I was always under the impression - or read or saw, regarding this subject matter - that Ford would deny your warranty claim ? if it was found out that you used
5W-20 in a 5W-30 application
or vice versa. 5W-30 in a 5W-20 application
Regardless of what was spec'd "the year before"
Again, if I am wrong, please specify, I am here to learn as well as un-learn mistruths.
Yes, Ford could deny the claim, but with the Magnuson-Moss Warranty Act, Ford would have to prove that the non-recommended oil grade caused the issue to be able to deny the claim, otherwise they would need to disregard that fact, or they would need to provide the specific oil grade for free. Some OE's, like Hyundai have pretty wide spreads in the oil viscosity they recommend depending on the temperature. Others just say "use this one, it's fine for all".