That's because the car needs a multi-viscosity gear oil... the transmisssion of most vehicles needs that kind of oil to cope with varying temperatures.

A higher SAE/viscosity rating doesn't also necessarily mean that it'll perform better in hotter climates.

GL-4 and GL-5 is also different, this isn't like motor oil where the preceeding rating is better than the last. If the transmission needs only GL-4, then just use it. If the transmission isn't made to take in GL-5, premature corrosion could occur.

Again, gear oil is different from motor oil.