Re: GL5 Oil question about the synchros.
I ran GL-5 oil in my trans for years. I run Penzoil synchromesh now. Transmission still shifts fine after 206,000 miles. I highly doubt your problems are caused by GL-5 oil run for a few hundred miles.
I agree though you should be running something else, either GL-4, synchromesh, MT-90, or the new Diaqueen.