I tried US1 2.5/5 wt as an experiment, and stock 919 forks were instant pogo sticks.
USF05 is a 5 wt, but I never tried it.
Factory oil is 10 W.
USF05 in F4i or 919 forks can only do one thing, and that is reduce the high speed compression and rebound damping force by some degree, as well as changing the damping force curve shape.
2004 + 919 forks will allow for low speed damping force correction.
F4i forks will do that plus allow for low speed compression force correction.
Worth an experiment for those that want to.
In one likes the results, then it's A OK !