can anyone recommend a good rubber preservative?

There's a product called Rubber Renue from MG Chemicals that comes highly recommended by UHF Magazine. Here's the page it's on :

http://www.uhfmag.com/Analog.html

As noted in another thread, there are a wide variety of natural and synthetic rubber products on the market. A compound that helps preserve one could potentially damage another or change its operating characteristics in the case of a driver surround.

Unless approved by the manufacturer, you'd be using a preservative at your own risk. It might work perfectly for someone else and horridly for you (or vice versa.) While it is certainly tempting to extend the life of your equipment, you are shooting blind unless you have some specific direction from the people who made the product.

a fresh box of condoms is like, what, six bucks? Just splurge.

But buy the box before you splurge.

That was the best laugh I've had in some time on this forum - thanks Kublakhan and Tarando!