I haven't looked at this in a while but in regard to synchros the GL-4 test requirement is for a certain level of corrosion related to components containing copper. A GL-5 oil that meets the same test should be fine for those components, however, there may be other properties of a GL-5 oil that are not suited to a tranny requiring a GL-4. I'm just saying it might not be just synchros to think about.