A few observations. Years ago, I used "Kal-Gard Smooth Stroke" fork oil, fortified with moly.... (who remembers Arco Graphite engine oil?) it worked great. 5 years ago I searched and found some old stock, but it was 30W. It worked great, BUT... when the temps got cool, it was a harsh, uncompliant fork (and I am DEFINATELY over 200 lbs,
).
20-50 engine oil had been recommended, so I have now (two seasons) swapped back to that. The action remains compliant in lower temps now. (the 30's is as low as I've ridden this bike).
The ATF post above is what Triumph used (T140 fork caps had a label on them), so maybe that's where that got started. I haven't tried it, but I'd expect it to be too light.