The switch to 20w/50 engine oil in the gearbox (as per the 1976 handbook) did indeed prove a great success, but not a long-lived one. The clutch worked well for about 80% of the trial but started to drag towards the end, making the Blue Hills 1 & 2 restarts tricky enough for me to mess them up completely. When I opened things up I found that oil had contaminated the plates once more. Not a lot, but enough to spoil the fun.
My latest thought is that perhaps fully synthetic is a bit thin for this application, so I have now stripped everything, cleaned it meticulously, and refilled the gearbox with good old mineral oil. It certainly appears significantly thicker. Hopefully the O-ring that seals the clutch drum will have an easier time keeping this at bay.
If this doesn’t work, I will replace the oil seal in the base of the clutch basket. It looked fine but you never know.