Jonathan,
I doubt that it's 'oil drying out' that's the problem. In my experience, it's overheating which will damage a coil, possibly by oxidising the insulating oil, as can be seen in the photograph below.
Accidentally leaving the ignition switched on without the engine running is the quickest way for that to happen.
.
I've got resin-filled coils on two of our Dyanes and regularly reverse the polarity of the low-tension connections, just as I've done previously with oil-filled coils, in order to equalise the wear on the plug electrodes.
There's never been any problem with starting and both cars will return well over 40mpg on a decent run out, so I'm not convinced by the advice that coil polarity is critical for these resin-encapsulated coils.
Primary winding of a 'cooked' ignition coil by slcchassis, on Flickr
