I used to use 80W and the Float Fluid in the chamber of two Suntour forks and never noticed anything seeping below the piston even after several months, so while I am not saying it couldn't have happened to others, maybe it's fairly uncommon?
I just believe the manufacturer should know what are they saying, considering significant amount of time was most likely spent on testing a fork before it goes to production.
Just look at what RS does with their air springs and dampers. They're changing and fixing stuff constantly. Some years bladders are superior to IFPs, some years IFPs are superior to bladders. Charger 2 comes out, then shortly after that Charger 2.1 comes out. They release an air spring where the seal had leaks air into the lowers, then they come out with a redesigned seal head, then they come out with another seal head that's completely different again, then they copy Vorsprung with their new air spring. Why should I trust that they know what they're saying?
RS only started suggesting adding oil to the air chamber recently. I highly doubt that the sludge leaking down is uncommon. Just think about it, you have a transfer port between the positive and negative.
But again, feel free to try adding oil, and make sure to report back after your next air spring service. 🙃