[JRTJH]

This statement is in the 2016 F150 Owner's Manual:

"For best overall vehicle and engine performance, premium fuel with an octane rating of 91 or higher is recommended. The performance gained by using premium fuel is most noticeable in hot weather as well as other conditions, for example when towing a trailer."

A number of EcoBoost owners have commented that they get better performance and better mileage using premium fuel when towing. I don't know how much improvement is "expected" and I can't say whether the improved mileage is offset by the increased cost, so it may be a wash to just accept the lack of maximum performance rather than pay the increased cost for premium gas. Since the OP's vehicle is not an EcoBoost, this may not even apply to his engine performance.