XC90 need a trans oil cooler?

Your car already has an ATF cooler internal to the radiator. Do you need an auxiliary cooler?

Nominal ATF operating temp. is 85-90C, the same as engine coolant. You shouldn't allow ATF temp. to exceed 106 C. They say for every 10 C decrease in ATF temp, it doubles the life of the tranny. If you flush the tranny at the correct interval, it has the same effect elongating lifespan.

Volvos pulling trailers are recommended to flush ATF every 30K miles (50K km).

Do you have a VIDA or Volvo-specific ODB-II scanner that can access ATF temp? That would answer your question specific to your driving conditions (ambient air temp, uphill hauling). Also the TCM software keeps a counter of the number of ATF overheating incidents, as a way to measure the "aging" of the ATF. When you flush the tranny, you want to reset this counter to 0 (which requires VIDA).