Gaerbox oil issue

I have a golf 1997 with 020 transmision.Bought Vavoline sae75w90 gear oil and it also states API SERVICES: GL-5. VW manuel says to use 75w90 GL-4.
What is the diffrence between the GL-5 and GL-4.Does it matter if I use the
GL-5 kind.Help Appre.
JO...:)