I have never heard of anything called API MT-1 nor have I seen it recommended by anyone for anything related to Jeep.For some reason folks seemed to think MT-1 means great for all stick trannies. Not the case. The designation API MT-1 denotes lubricants intended for nonsynchronized manual transmissions used in buses and heavy-duty trucks.
GL5 oils have a sulfur content that that destroys softer metals "yellow metals" as many call them. That would be the syncro rings and possibly bushings in some cases. GL4 and down does not have the damaging sulfur thus the recommendation. There are some manuals that now use carbon fiber syncros andIi dont believe the GL5 will do damage. I think I mentioned it that the likes of Summit racing or Jegs and other do in fact carry GL4 lubes as well as synthetic ATF.