What about deleting the oil - water cooler and going with an external oil cooler? That would free up cooling capacity in your engine so it can absorb the excess heat as you pull a long grade.
F-150 EcoBoost Oil Cooler Installation Guide • Full Race
F-150 EcoBoost Oil Cooler Installation Guide Start by bolting the lower mounting beam to the front bumper support, then bolt the oil cooler to the frontwww.full-race.com
Im running a setrab oil cooler in series with the oem cooler. I was worried that at low speeds the setrab might not provide enough cooling. I have been climbing some really steep grades at slow speeds before and had my coolant temps spike. Climbing from Panguich to Brianhead here in Utah is like a 4000’ climb at like 20-30 mph and i had the cabim heat on max to keep the thing cool.
I mounted it directly in front of my intercooler down low so that it is in a separate air stream from the radiator.