I've ran a used one for 100,000 plus miles and I can't feel any difference from the stock part I took off.
I tried to figure this out years ago with no luck.
I'm looking forward to possible answers.
No answer to that from me - just my own observation. I replaced mine last year at about 60,000. OEM brand new. I checked the amount of slack in the adjuster, and had to top it up - it took 9 clicks before it met any resistance. After a top up it reduced to 2.
The difference that the new shock made was remarkable. The bike felt planted on the road, much more stable in the bends and it is riding really well. Seems to be working well with the T32GTs.
I didn't realise that it had been riding badly - I ws quite happy with it as it was - but decided that I'm probably keeping this bike now until I stop riding, and it would need a new one at some point in that time. I might as well get the benefit from a new shock while I can, rather than fit it later and only get 10,000 miles from it.
The preload adjuster slack is a red herring - that amount of slack won't make any difference until you get to the point where it won't compress the spring any more. Then the extra fluid would provide 7 more clicks of pre-load.