Antony,
I had a similar issue using 5W-40 synthetic. The issue was mostly resolved by changing to 20W-50 Valvoline VR-1, a petroleum-based oil ('dinosaur oil'). The VR-1 formulation has enough ZDDP to support 'flat tappet' motors (although US emissions is cracking down on ZDDP content). There are other dino oils with high ZDDP content (Motul, others). The trusted engine builders here recommend 20W-50 weight petroleum-based oil with higher ZDDP levels.
I went through 4 oil seals on my current motor (built with parts from two motors, over 50K miles since August 2012) trying to solve the final small leak problem.
I am currently rebuilding the original motor:
Ol Smokey Rebuild. Both motors use Scat crankshafts which I hope is not the source of nagging main seal leaks...
Peter