I've been messing with the fork oil in my G5 this afternoon. I've been using Silkolene 10w fork oil, but lately I've felt the bike has been diving a bit on heavy braking. I'm 160 pounds, so not a heavyweight. I tried Silkolene 15w, expecting only a slight difference, but the forks barely compressed at all and the ride on a bumpy road was horrendous, rock hard. When I got back from my test ride I realised I'd put 300ml in each fork instead of the recommended 200ml, so perhaps this was to blame.
Finally I mixed a quarter 15w to three quarters 10w, and put 200ml in each fork, and it seems about right. A longer ride tomorrow will tell.
I should add, re the diving, that my front brake has always felt a bit grabby, so I fitted a set of Ferodo organic pads from Hitchcocks at the same time. Big improvement.