Undoing what previous owners have done (which I call 'unf'ing') is a large part of what I do when rehabilitating/restoring older trucks. (Some people's 'repairs' are a wonder to behold ...)
Short of running it dry, it's almost impossible to destroy a Dana 70 ... after a million or so miles, the gears and bearing might wear out ...
Unless you're putting in a lot of miles in a hot desert, I'd run 75W90 gear lube. I use 75W90 in all my gearboxes/differentials because I run year around in severely subfreezing temperatures.
Regular or synthetic is up to you. (I haven't made up my mind as to which I prefer.)
