there is a special lube that is used for gaskets. It's called Fomblin grease. It's the best of the best but it is not cheap. This is what rolex uses as well. It's about 50 bucks for a small container.. using anything else like the cheap lubricants like silicone are not meant for this type of application.. and WD40 is a NO NO.. fork out the cash for the right stuff or send it into rolex.. You will only make the crown and case gaskets worse by using other agents.
http://uk.rs-online.com/web/p/greases/6664903/