Hi Mike,

I use Wurth rubber "conditioner" that contains glycerin, not silicone. Comes in a spray can. I also bought liquid glycerin from an internet "chemist store" which I use regularly on rubber hoses, door seals etc. It works best for maintaining good rubber rather than resurrecting old rubber.
Wurth would be your best bet, around $11 per can if memory serves me right. I order 3 or 4 at a time having 3 cars and a motorcycle to take care of.
Good luck
Reinhard