I replaced the OEM fork springs on my FZ6-S (2006) with progressives from Hyperpro (good brand, not too expensive) which give a much better feel as the originals were made soft for comfort, rather than handling.
AIUI, progressive springs are better for on-road use because they are designed to deal with both small bumps and big pot holes. You wouldn't find these on a track, so linear springs are a better option for consistency.
Thicker oil just increases the damping, meaning the front end would be slower to dive, but also slower to recover which doesn't necessarily improve the handling as it can be less smooth on bumpy road surfaces.
Having had a look at the FZS 600 fork construction, it's very similar (if not the same) as the FZ6 where there's just a simple spacer (parts 16 and 35 on here:
https://www.bike-parts-yam.com/yamaha-mo...S600/24252 ) which gives a set amount of preload for the "average" rider, however that may not work for you if you're not an average weight.
Once I'd put the new Hyperpro springs in, I looked at videos online for how to work out the correct preload for me, then cut two pieces of PVC tube that would fit in the fork tube and bought some penny washers.
I measured the tube to a shorter length than the stock spacer, then kept adding penny washers to increase the preload (as I only weigh 63kg) and checked the free sag and rider sag until I got the correct amount for me.
Then I used a pipe cutter to shorten the spacers to the length I'd worked out and I was sorted.
NB unless you're doing a lot of Track Days or racing, IMO it's really not worth buying the preload/ compression adjusters that you use to replace the bolts on the top of the fork tubes that let you make mini-tweaks to the settings.
Hope this helps.
PS I swapped the rear shock for one from YSS which is also much better than the OEM one.
