Not a Boxster-specific answer but most gear oil is designed to operate best between 160-220F. Below 160 things can get a little stiff and above 220 you lose viscosity and risk damage to seals and bearings.
That said I agree with Tom that 90% of trans breakdowns are driver related. I tossed the pinion bearing in mine with torque spikes while getting airborne on the whoops running the historic track at Pomona. Fun but expensive.
