I work in automotive electronics, and for long-term reliability we call time at 80 to 85 and go into the lowest possible power mode. We would expect up to 70 behind a dash in summer. 50 is really the comfort zone if you can achieve it. Whilst the silicon may be able to handle 80 to 90, you need to consider the long-term effects - Not only actual temperature, but also thremal cycling between on and off. Over time, it can crack solder joints and cause internal stresses, leading to eventual failure.