Once the protective clearcoat fails, the lenses will fade, no way around it. The restore products work great but require elbow grease.
I'm lazy. To quickly and easily remove the yellow oxidation, spray a cloth rag with any brand (or scent for that matter) bug repellant and wipe the lens. I don't know how, and I don't know why...but it instantly removes the oxidation. Takes 5 seconds per light. Then follow-up with a good coat of your favorite wax or polish. Kinda frightening that we spray this on our skin! I use a generic $3 aerosol can from Target. But I've also used "Off" brand backwoods scent, and unscented. Also used Cutter brand.
For ****s n giggles, I am experimenting on a friend's toyota to see which product keeps the lights from turning yellow again. She bought the Turtle Wax restore kit that comes with a product that you wipe on after cleaning to prevent future fading. We restored one headlight following the kit's instructions (word for word). Wiped the other clean with bug repellant. On the bug repellant lens, i wiped it after cleaning with BananaBoat 30spf sunscreen. Two weeks today, so far neither seem to be fading.