lubing the sliders will increase smoothness and because of this may quiet down the sound. If you have a BNIB Realforce I don't find this necessary, maybe after a few months or year would you really want to lube the sliders and regrease the stabilizers. I recommend Krytox mixes (103/205 if you can find it), I have used
thick Geekhacker's V3 lube and it works wonderfully.
The Realforce with purple sliders feels pretty good, the purple sliders are designed with shorter slider "legs" to accomodate the silencing ring and provide the same amount of travel as a stock slider. The black sliders in the Type-S are meant to be the same but their legs are longer than a purple slider, with the silencing rings this will reduce key travel.
Personally I feel that PFU/Topre have the silencing backwards - IMO the purple sliders work best in a HHKB because the extra travel in those sliders enhances the feel of the switches being case mount; on the other hand, the slightly thicker silencers found in a Type-S work better on plate mount Topre boards for almost the same reason, to me the feel resonates better on the metal plate and feels more satisfying to type on.