We used some 1/16" Metal Shield MC-706 with a 90/10 that is similar to the ER70S-6. It dealt OK with light mill scale, but FCAW deals much better with it.
For some unknown reason we had sporatic porosity issues that we never could resolve. Had the Lincoln rep in here and everything. He thought it may have been spots in the roll that didn't have enough in the core or something....we could pull off 20-30 feet of wire and the porosity would go away until we put on a new roll and then all of a sudden it would return. Replaced everything from the tip/nozzle back to the bottle and never could eliminate the porosity...we gave up and took the wire off all of the machines and went back to Ultracore 71 and all of the porosity problems went away. I still had a few rolls left and decided one day to try and use it up....put in on just one machine and within the week we were gouging out welds due to porosity again.
I like the way it ran and welded but you could not predict when it would blow holes. I liked the puddle control and how you could get it to wet in, bead appearance, operator appeal and all....may give it another try in the future, but the plant manager gave me a fit until until I took that stuff off all of the machines.