Volvo reccomends transmission services on all models at the 52,500 mile interval if used for towing. The dealership I used to work at didn't do too many services on S80s. They usually don't need a service until 90-120K. It all depends on the condition of the fluid. We'd do 850s every 30K, eventhough it wasn't reccomended. The ones we never did were the 5 speed automatics, that fluid is like $16 a quart!