I bought a 3oz tube of Permatex 22058 Dielectric Tune-Up Grease for spark plug boots years ago (and I expect I’ll be able to pass it down to my theoretical grandchildren someday at the rate I’m using it) and it seems to work quite nicely. It’s silicone based and has worked for me, so far. Also great for my pocket knife (a Kershaw Blur; I use it in the pivot as well as in the recessed spring ‘well’ inside the handle).