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Hi.
Fortunately, it seems your trip won't involve any climbs in elevation, all of FL being near sea-level :-).
I think you've covered all the bases -- I like using synthetic lubricants, because they don't break down in the heat, but 20W-50 should be fine, especially assuming (again, you didn't say one way or another) no turbo [for turbos, synthetic is the only way to go, to prevent coking in the bearings and to handle the extra heat, though.]
The reason for avoiding overdrive is to keep the torque available to the drive wheels higher: an overdrive gear, less than 1:1, literally reduces the overall drive-train's ratio to less than that of the rear axle, and therefore lessens the torque to the rear wheels than it would be in a straight 1:1 top gear. By avoiding overdrive, at least you still have the rear axle's ratio, and this greater amount of torque lessens the load on the car's engine, and prevents overheating.
Think of it this way -- being in overdrive is like having the car constantly climbing a grade -- you need more power to keep moving it along -- and pulling a trailer (with its mass as well as its greater air resistance) just adds to the required effort, as if that grade is even steeper. It's no wonder why the car would tend to overheat more than in a top 1:1 gear.
Additionally, you know the horsepower curve -- the higher revs keep it more in the power band; and you lessen combustion chamber pressures (avoid "lugging", although that might be a slight stretch).
Consider the alternative of driving a car pulling a load in overdrive -- somewhat higher revs is not that bad (assuming you've got good quality oil) if the engine is working easier. And while it's reving higher, the oil and water pumps increase their flows, the fan is turning faster (if it's needed, viz. the fan clutch); in all, a much better situation.
Good luck.
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