I've used the LAST record preservative since the 1980s, always after cleaning the LP first (mostly with a Nitty Gritty vacuum cleaning machine). I don't recall the preservative being claimed to improve the sound of the record, just prevent wear--which would maintain the LP's sound quality. On some of my LPs, I think it added low-level surface noise; after realizing that, I've been reluctant to use it on LPs with quiet music.
I've not done any careful comparison of treated and non-treated LPs. Pretty much all of my LPs that I bought new still sound fine, whether treated or not, in part because of using an antistatic gun and carbon-fiber brush before each play, with the record under a direct light so I can see any visible dust particles. I rarely wet clean them more than once. I store them in rice-paper or poly-lined sleeves. I play them with the dust cover of the turntable down. All of this effort to minimize dust collection on the LPs pays off in fewer ticks.