Horizon,
I reckon the high detergent action could possibly be the problem by releasing any carbon/crud build up around internal oilways in the rods,crank,camgear and galleries.
Very difficult to prove though.
Out of interest, I have run a 1936 vintage Riley 9 sports/racer on 10-40 semi-synthetic and two classic 1960's race cars on 10-50 fully synthetic without any problems for several years.
Anyway,I wish you good luck with sorting out your problem as it all sounds a bit horrendous!
Lister
