Update: I talked to the Mercury Racing tech guy last week, and now Mercruiser is recommending Kendall 20w50 on the 600sci ($35k), the 850sci (80k) as well as the 1075sci (over $100k).
I'm four hours away from an oil change, so I'll post the results of my UOA when I get it.
BTW: Our local Mercruiser Racing mechanic has a couple of 598ci 780hp single carb beauties in his shop to be installed in a big Fountain. I asked him what oil the owner was going to use... Kendall GT1 40wt. The mechanic has a store room full of the stuff...