I have not done any UOAs to see what the effects were, but on my 3MZ-FE, it has run mostly 5W-30 over it's life, a leftover blend that included 5w-30, 0W-30, 5W-40 and a touch of 5w-20. It has also run 5W-20 and as of the last change, I think I will be sticking with 0W-20 for some time.
Never ran anything substantially heavier than a 30 weight, the blend I used would have equated roughly to around a 32 weight.
No scientific data to say if one viscosity was better or worse. But my engine is still running nice and strong.