Coming from a fox fork previously, I tried a couple ml of float fluid on top of the piston when I rebuilt the lyrik. Bad idea! It moves to the negative chamber like Vorsprung explained. It makes it super harsh, the fork felt like crap. Rockshox recommend to use only grease instead and I would avoid oil altogether in there. FWIW, it felt like it made the seals swell as well, the piston didn't move freely when moving it by hand with the air spring disassembled.
If there is no oil in there and every thing looks good, I would look at the o-rings around the baseplate and shaft. Basically the same ones as for the OP, but on the spring side. It's a pretty basic air sping, there is not much that can go wrong and you said you looked at the valve already.
Hope it helps
Good advice in general. A note on this - there is only one oil I'd recommend using to lubricate air spring seals and that's Float fluid, but all you want to do is wipe a bit of it on the sliding surfaces (seals and piston/bushing), not pour it in there (except in the case of Float forks as you mentioned), as all oils do interfere with the volumes of the respective air chambers. If you don't have access to Float fluid, slick honey grease is the other option (non-silicone-based greases also not recommended). Other suspension oils are much too thin to do the job and will result in things feeling comparatively sticky. However, over time grease simply gets pushed out of the way and stops lubricating as effectively, so it tends to be a shorter-term solution than a lick of Float fluid.
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