I had used the Repsol 5 weight first so the transition to Amsoil 5 synthetic wasn't so dramatic. The 5 weight makes the fork action softer. If it bottoms, just raise the fork oil level in small steps until it stops bottoming. I made the change to Amsoil when I installed Ricor valves which replaced the Race Tech Gold Valves which I sold to another NT rider. Both types of valves are an improvement but the Ricor's have an anti-dive function under hard braking which I love.
I used Progressive fork springs (for PC800) for a while then went back to the standard NT springs but kept playing with different length spring spacers. In the end, what worked best for me were spacers that were actually just below the top of the fork tube with the fork fully extended. At least in my experience, the standard NT fork has way too much spring preload so the front end often jumps over small bumps, rather than compressing the fork. Even with the drastically shortened spacers, the laden sag (rider in place) was still OK which supports my theory that it had way too much preload.
Once you get the fork sorted, it'll make the rear shock feel too harsh which it is because the standard spring is too strong for someone who rides solo and weighs less than 200 lbs.