Drive it a bit, and the diff should settle in. I wouldn't want it completely open. You will end up losing exit speed coming out of the corners, having one wheel spinning all your power away. There is always going to be throttle control coming into play in getting the optimal cornering speed. So...
If you put thicker diff fluid in, that's not helping with oversteer. Try thinning it down.
As Chewy hinted at, if you can adjust the camber, I'd try adding a little more in the rear and see if that helps.
Does the Magnum transmission housing have the same angle on the arms to create the toe in...