"GL5 is a different spec, and is not 'backwards compatible' with GL4.
GL5 gear oil is a lubricant with very high extreme pressure performance needed for Hypoid gears where sliding and rolling contacts operate under very high loadings. This is achieved by incorporating high concentration of sulfurized additives. The downside to such high sulfur in oil is it tends to corrode yellow metals, so can ruin parts like brass synchronizers and some bushings. So any equipment needing GL5 performance is designed taking this limitation of the lubricant into account.
GL4 oils are much more friendly to yellow metals, and so are used in equipment where such high load sliding-rolling contacts aren't present. The designers of such equipment aren't as limited in the use of brass parts. So the bottom line is that if your equipment calls for GL4 it probably doesn't need GL5 performance, and you may be asking for trouble down the line if you use a lube it wasn't designed for."
Stick with what's been specified by Land Rover..., saves any future issues.
