I would look underneath and see if you can See oil seeping or leaking from the upper oil pan. It is a common problem and sometimes hard to detect because it may not leak much and may not drip on to the ground. If it has that leak, it is 15hrs of shop time to replace the faulty factory sealant. The transmission has to be removed for the repair. Very expensive. Also, look for signs of corrosion around the 7 pin trailer harness. There are 2 harnesses that should be checked as well on the passenger side frame rail near the spare tire. These are know problem areas and I wouldn’t buy a used Superduty without checking these areas.