Higher temperatures increase a grease oxidation rate, typically doubling it for every 10°C (18°F) rise above 65°C (150°F). Grease softens as temperatures increase and may become fluid enough to leak out of housings. And oil separation and volatility also increase with temperature.
Therefore, high temperature operation requires relubrication more often; high-temperature greases that have the capability to maintain structural stability, oil separation, volatility and oxidation under control can help extend that frequency.