I'd also recommend changing your oil every 3-4k miles if you continue noticing that sludge, even if you're using synthetic oil. Synthetic oil can safely last 5-6k+ miles under normal conditions, but if you're experiencing condensation accumulation, you'll want to change the oil a bit sooner. The water in your oil is hard on the bearings and other friction surfaces, and the more sludge you build up, the more chance you have of clogging an oil passage, etc.
1998 BMW 328is
1966 Pontiac GTO
2007 Subaru Impreza 2.5i 5-door
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