I did some checking. The AGCO 80W-90 that we used to sell at our dealership was a GL-5/ I checked a jug at Car Quest, and theirs was GL-5 as well. So the short answer is, I have been using GL-5 for some time now, with no ill effects.

I also checked my 77, and yes, the wire tube does bolt to one of the large rear axle bolts that go into the rear end. They changed the design with the Super series, and that eliminated them being bolted with the large axle bolt.