Use Baader Ultra-Range Machine Grease #4 by Alpine Astronomical to treat sliding mirror fittings of your telescope. This synthetic grease is rated for a wide temperature range (-58 to 140°F) and will keep your telescope parts in good condition for a long time. It is designed to maintain its viscosity throughout a wide range of temperatures, will not release any gases or vapors, and will not harden or separate into solid and liquid components over time.