I mix m1 0w16 with pz 20w50 in a 50 50 ratio. Smooth like butter. No harmful effects. This is my summer brew. Cheers.
There won't be any "harmful effects" if they are API licensed oils, that should not be the focus of the discussion. Successfully passing ASTM D6922 demonstrates there will not be harmful effects.
But the mixture you made may not necessarily meet any specification, license or approval including SN (and likely does not).