I've used GL-5 conventional for both for years. Transmission has always made a grinding down shift to second only when cold. I've heard nothing but great things about the MT-90 as a GL-4 so that's what I'm switching to this winter. Everything else (transfer, front diff, rear diff) is getting the Lucas heavy duty gear oil. I doubt a GL-5 does any damage to the transmission, but I've just hear so many great things about the Redline.
So don't worry. You aren't the only one over thinking it. For how much we all end up spending on other nonsense throwing a few more dollars at a fluid doesn't seem unreasonable.
This was my Black Friday gift to myself