I called and talked to Dave Granquist at Redline, he is their tech guru over there. As soon as I mentioned that the factory required GL-5, he immediatly said 75W90NS from the trans. I asked about MT90 and MTL, he said they are both GL-4 and when the manufacturer calls for a GL-5 in the trans, use it... simple as that. He didn't seem to know a whole lot about the trans, since it also shares fluid with the front diff. The need for GL-5 may stem from that. He also said straight 75W90 for the rear diff, wheter it has an LSD or not. So, from Redline's recommendation and the fact of saving my 6/60 powertrain warranty... I will use 75W90NS.