Interesting info. I typically call it quits around 25f, mainly because the bike feels so crappy that it just isn't worth it to get out, days that cold are regular for only 6-8 weeks of the year here, so I can usually afford to spend the time in the gym instead of on the trail. The suspension feel is only part of it, though, I notice a difference in tire traction as it gets colder and my whole body just feels stiff and uncomfortable.
Interesting note on the Manitou seals, do you know if they used the same material on the Mezzer seals?
I considered trying new fluids, but we have a wide range of fluctuating temperatures during the winter. It'll be 55f+ one day, then a few days later down to 25f-30f. It means all winter I'm having to check suspension pressures before every ride and I've been nervous that changing fluids would not play well with fluctuations that significant. TBH I haven't really looked into it, though.
I tried a few forks last winter and had mixed results. The Auron I ran really started to feel poorly below 40f, it became noticeably harsher and slower. Backing off the adjusters provided a minor improvement. My Ribbon didn't change much until about 30f, same with the Helm. My DVO Sapphire would start to slow down and get harsh around 35 or so.
