Hey Cosworth, I cant help but explain what and why. ;D
Motorcycles have been my 'life' ever since I can remember. my father ot me started when I was about 18months old (so my mother used to tell me)
Apart from 8 yrs working on construction equipment it's the only thing I've ever really done. (and even then I was still 'playing' with bikes)
I trained as a precision machinist so I could make bike parts, fitting BSA bits into Royal Enfield's, parts from different bikes modified to fit something else. The first 'big bore' conversion I did was 1975
I worked in a couple of Yamaha dealers, probably ridden more bikes than you can imagine.
I've ridden SR250 and XT250, (and XS 250, 400, etc) The forks are soft, as were all the bikes designed for California market in the 60's and 70's(and most of the 80's)
I ran dealer workshops but even before that had 'free reign' in all the dealerships I worked at (Honda, Suzuki, Triumph, BMW, Vespa, SILK)
Ive had to pick up plenty of crashed bikes, sometimes serious, mostly 'trivial', I always want to know 'WHY' something happened
Thick oil didn't compensate for soft springs 30+ yrs ago and doesn't compensate for them now.
What really happens when you fit thick oil is it prevents front end re-bound if you hit a series of bumps or undulations
It's a pretty well known phenomenon known as 'packing'
I also raced motocross and enduro
A far better way to increase spring rate is to cut a section off the closed coils of spring and fit a spacer. Fitting a second spring makes forks softer as you have more working coils.
Your using bikes as a hobby to make cash to support hobby you should start studying what your doing instead of following the herd then recommending some cheap and nasty 'solution'
I'm not trying to piss you off, just explain I do know what I'm doing