Hello - Southern California Jetta TDI Guy

If you are getting some drastic temperaturte changes you may get a little condensation and a patena of rust developing. this is not problematic rust though, 50 MPG is right that it will wash off after you fire the engine up again. if you want you can put some oil in a spray bottle and give her a squirt to keep the area coated with oil, that should keep the rust bugs out. Just clean the gasket sealing surface well before reinstalling the gasket if you do this.