Thanks for the confirmation of 139mm, at least I'm not going mad!
I'm assuming the forks were underfilled from the factory.
I reckon the mix in there is 6.5-7 wt, a quick run out yesterday showed a definite improvement in ride quality from the front, I had a short run on a local cobbled & potholed track, the bike rode much better than when I tried it in summer when it felt quite horrible.
I wouldn't hesitate to go to 5wt if doing a change although this would be with the emphasis on ride quality maybe at the sacrifice of spirited riding stability.
Phil
Edit, re reading my original post, it's a bit information overload.
Main points:
Stock oil is 10wt, air gap in my bike was 178mm
Current oil ~7wt, stock air gap of 139mm
Not all fork oil of a given wt are the same viscosity