There is some conflicting information on the matter, which may be due to the additives in the oil, but generally experts say that conventional oil has a life of ~10,000hrs at 140°F/60°C and every 18°F/10°C rise above that cuts the life in half. This of course assumes the limiting factor is oxidation and not a contaminate like dirt or water.
158°F ≥ 5000 hrs
176°F ≥ 2500 hrs
194°F ≥ 1250 hrs
Vickers Mobile Hydraulics Manual, page 372, section 'Heat or thermal contamination': "Their useful life, as well as maintenance of viscosity, chemistry and the other attributes is based upon continuous operation below a critical temperature. This critical temperature is 140°F. Every 18°F increment higher than 140°F effectively doubles the oxidation rate of the hydraulic fluid (petroleum based) thus cutting its useful life in half, e.g. running a system at a consistent 176°F would reduce the useful life of the fluid by 75%."
Oil Hydraulic Systems, by S.R. Majumdar: "As a working rule oxidation rate is doubled for each 10°C/18°F rise in the temperature. An oxidation-inhibited high viscosity index (HVI) oil may give a useful life of ~100,000 hours at 40°C/104°F even under the condition of aeration and catalysis. When the oil temperature is raised to 60°C/140°F, life drops to as low as 10,000 hours."
Synthetic oil doesn't start to break to till at least 170°F or higher depending on the base stock and additives
Hoses and other rubber components are generally good till at least 212°F/100°C depending on the compound. But similar to oil, the hotter they run the shorter the life.
ISZ