Trailer weight is not normally a problem, until you factor in the weight added to the car by the number of pwople and what is in the trunk. You live in a hilly area, so you might be forced to down shift to 3rd on very long hills.

You also live in an area where the temperatures do not get over 90F (30C) much, so the current radiator design to cool the ATF and engine oil is quite good, even with the AC running. The turbo already has and oil cooler.

So, to answer your question, no. But you should flush the ATF more often if towing through the hills becomes routine. I would switch to the more expensive Mobile 1 ATF or another synthetic ATF compatible with Dex/Merc fluids needed for the 4 speed automatic.

Too bad there are no displays for the ATF and oil temperatures. You might want to contact IPD in Portland, as they have a gauge holder that inserts into the driver's A pillar and can accomodate 3 gauges. (Voltage meter?)

With your current setup (stock), the tranny will drop into 3rd gear if the ATF gets too hot. This increases the ATF fluid pump speed to help cool the fluid. If this happens, you should also turn off the AC until the tranny again allows 4th gear.

There are stories of folks driving up the pass from Oregon to California with an overloaded 850, the tranny downshifts to 3rd gear until well after the top of the mountain pass.
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My back feels better when I sit in a Volvo seat