A synthetic 10w-30 motor oil will work fine for transmission fluid, and wont eat your brass synchros like some others will. Mobil1 seems to be a popular one.
However, most people will swear up and down that Redline MT90 is the be all end all. Jerry Bransford advocates it, so I'm inclined to believe the hype.
If, when you drain it, it looks like liquid gold, you're in trouble. That means the fluid that's been in there forever has eaten through your synchros. But that's highly doubtful, as it's probably mopar fluid and midnight black.
In case you're interested in what we're talking about/how manuals work...