I have always rebuilt mine using "brake grease" on the seals & boots as well as the piston & piston bore
How does the brake grease differ from other petroleum based greases or oils?

Curious as most greases or oils are good at keeping bare steel from blushing when they get wet from temperature changes. Stored parts in the NE usually end up seeing some big swings in both temperature and humidity unless you're keeping them in climate controlled areas which might be why the parts stores do OK w/ the calipers just being bare & dry.