So far, in over 2 years, I've used nothing. Cover and pillows get regularly cleaned, but I'm not using any preservative stuff. Cover and pillows still in great shape, but my spa isn't subject to much direct sunlight, and whatever my water regimen is it isn't degrading the pillows.
I've heard the Armor All isn't recommended, or that once you start using it you have to keep using it forever. No experience with that, just repeating what I heard.
Brewman