303 aerospace is probably the best stuff to use. It helps to block ozone for a while. You cannot get 303 at autozone, most RV or marine places sell it. Some people use silicone, but that attracts dirt/dust.
If you read through the many uses of 303, the site mentions using it on CV boots. BTW, this stuff if awesome on all the dash and interior trim in our cars. It really is SPF for your stuff.
I dont know if you are near a Cabelas, but they sell it for a decent price in the marine section.