LT230 - GL4 or GL5

evening all, I know 75w90 GL4 is preferred for the transfer box however it seems quite difficult to get hold of cheaply. Is modern GL5 ok to use now? I’ve read the formulas have been updated, but I’m not sure how to tell if it would be safe or not.

Smith & Allan do it but state both GL4 and GL5, whatever that means? Even Ashcrofts say to use either
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/371892205853

I use GL4 because I have an overdrive unit (which shares its oil with the transfer case) and GL5 is not recommended because there are bronze components inside and GL5 is quite aggressive with them apparently. I've found the easiest thing is to just get it off Amazon, where you can get 5l containers for a little over £20.
Amazon product
Maybe I could find it cheaper if I spent all day going round motor factors and agricultural merchants, but it wouldn't be worth the diesel I'd use up doing so.