Be careful of modern oils and modern anti friction additives. Word has it that some of them reduce the friction too much and make it difficult for the synchromesh cones to bite properly so they heat up and wear out. That's the main reason why its GL4 rather than GL5 that's specified as GL5 has more anti friction additives. I'd assume they wouldn't do well on Slick 50 or Molyslip either. I took the GL5 back out of my main gearbox and overdrive and replaced it with GL4 when I found out about that. I left it in the diffs, hubs and transfer box though as they have no synchos. There also used to be an issue with some oils attaching brass components but I think all reputablel modern oils are buffered to prevent that now.
Disclaimer
That's my understanding of it from what I learnt on here and researched a bit further on the web. It could be wrong in whole or in part but i'd suggest that its worth considering and checking out before you change from the original spec oil thinking its an improvement.