The ECT/EOT delta that everyone references is in a Technical Service Bulletin (TSB) issued by Ford. I would search for it here but I'm on the mobile. In a nutshell it says truck fully warmed up (190+) at 65mph, flat ground, for a mile. If delta is greater than 15deg change oil cooler. It's been a while since I read it so someone correct me if I'm wrong. So a 15deg delta isn't normal, it's time to order parts or make an appointment with your mechanic. Checking the delta under other circumstances like cooler engine, different speed, up or down hills etc. Might give you an idea of what the delta might be but why buy gauges and then fudge it on the details?
The max delta figure in the TSB is from some engineers who likely considered alot of factors and Ford is in the business of making money. I don't think they would recommend changing it one degree before it was necessary.
My delta following the details above exactly, are 8,9,or 10 degrees. According to Ford I don't need an oil cooler yet. Towing our nearly 10k fitb wheel I've seen ECT as high as 225 and EOT of 240. The truck is in spec with the TSB but I plan on changing both coolers over the holidays just because I don't like it. I've read oil can go over 300 deg without scorching or what ever and full synthetic can go much higher but I hope to never see those kind of temps.
Also, alot of guys who change oil and EGR coolers report delta's of 3-5deg on the new coolers. there was even one member who reported a 0deg spread! I think there may be a degree or two tolerance in the senders. you can get an idea or the tolerance on your truck by looking at the temp spread after a long cold soak.