There was a product called Penetrol. If you try it, do it in an area you can't see to test. I don't know if you will find much. Most will be a petroleum based product and likely will evaporate over time. Call me crazy, but I have an Azek deck at my summer place which is about 5 years old. I've noticed the fade pretty significantly. I tried various products without success. They look good for a while, but always go back to the faded color. All except one - bug spray. My kids blasted their feet year one which bummed me out as the deck was new. After all this time, the color around their foot prints is dark and I can still see their little foot prints. I'm tempted to try wiping in down with the stuff at some point....