My bike is now exactly 2 years old and done close to 9000km. The stock damping in the forks had basically become non-existent, and were bouncing up and down even with the rebound tap completely shut off clockwise. Since I had some Motul Expert Medium 10W lying around, I decided to completely ignore Forks-By-Matt's advice and change out the oil today to the 10W (which is about 36cst@40 deg viscosity). I only swapped out the left fork oil.
There was about 390mL of oil in the left fork, so I put back 390mL of the Motul Expert 10W. The stock stuff came out like apple juice. The whole job took about 2 hours with a Park bicycle stand for holding the fork, and regular hand tools. (8/10/12mm sockets and spanners, 5/6mm hex, 14/22mm spanner, 14mm hex socket for axle, torque wrench, 500mL measuring cylinder, and plastic funnel). My subscription to Dave Moss Tuning videos helped with preventing any derpy moments.
Verdict: 10W / 36cst viscosity fork oil seems fine. Slow speed rebound stabilises at around 3 clicks out, but high speed rebound (i.e. fork overshoot) stops at around 1.5 clicks out. Low speed compression seems better supported, but still reasonably plush. It's not perfect, but a hell of a lot better than the stock damping, which started to die around 5000km anyway. FYI my preload is 14 turns in for a 90kg rider (35mm front loaded sag).
Meanwhile I've got a Nitron NTR R1 rear shock and TVT cartridges on order. The cheapo oil swap will keep me satisfied in the meantime.