See Tanis preheater frequently asked questions about bullet point 15.
https://www.tanisaircraft.com/faq/
Here is another viewpoint.
Condensation inside of an engine case happens when the temperature of any part of an engine is below the dew point of the air in the case. This condensation enables corrosion. Condensation can be controlled and it can be manufactured.
It is controlled be using a dehdrator. It is manfuactred when warm moist air is maufactured but parts of the case are colder than its dewpoint just like a glass of cold lemonade in the summer. A dehydrator will lower the dew point to keep the inside of the case dry..
Leaving any preheater on that cannot warm the entire engine above the dew point is likely to cause condensation inside the case. So, if the oil pan heater is warming the oil but the cam at the top of the case is still cold it can precipitate condensation. But If the entire engine temperature is kept above the dew point science says condensation will not form. Using a timer to Cycle a preheater to turn on every morning for a couple hours will increase chances of condensation. If you fly everyday it’s a great idea but If flying once a week it’s a convenience that may be forming condensation on a daily basis inside the case.
So its a good idea if you use a hair dryer to blow hot dry air through the oil filler port to purge out the breather tube the engines moist air in the case that exists after flying. If you hook up the dehydrator before you close the hangar door, youll lower the dewpoint inside the cast to help avoid condesation. If you turn on your prehetaer on two hours before flying then you can sleep well at night even if you don't fly everyday knowing scientific principles are keeping your engine case dry.