Try another brand of oil next go around. I was getting the opposite problem, viscosity loss, using a Delo 5w40 synthetic. I switched to Schaeffer synthetic oil and the unexplained viscosity loss disappeared. Over the years, I have become convinced more than ever, that engines, even within the same brand, can be fickle about what oil they seem to prefer. Not that the oil or the engines are bad. It is akin to .22 rifles and ammunition. I can take two high quality rifles and feed them both a quality ammo and get one that shoots better than the other. If I change to another quality brand of ammo, the results reverse.
Try a different brand and see how it plays out on the oil sample next time.