I've seen 2-stroke oils gel up at the bottom of the reservoir
tanks more than once. I don't know if the oils were mixed, but I know
some of them had been sitting for an extended period. The Mercury
"on board" reservoir tanks like what's used with the V6's have a pickup
tube with a fine mesh screen at the bottom. I've seen those clogged up
with a thick, parrifin feeling coating that blocked oil flow.
When we had all Merc 2-strokes in our rental fleet, we used the standard
Mercury 2-stroke oil and never had any issues with jelling or oil related
failures. IMO, using a synthetic or blend 2-stroke oil with a carbureted engine
isn't necessary. DFI engines are a different animal and I'd use whatever is recommended
by the MFG.
An above poster is correct when he said that if you mix to 50:1 it will be lean on fuel.
You may research if there was a jet change recommended when switching to 50:1 ratio fuel.