80W GL-4 and GL-5

I recently bought a used Kuhn GMD 700 HD hay cutter and luckily got the owners manual with it.  I want to change the gearbox and cutter bar oils.  The owners manual calls for 80W GL-4.  Most of the local parts stores carry 80W90 GL-5.  I looked on the internet to find the difference and it appears to be the additives and cautions against running GL-5 when GL-4 is called for due to possible corrosion of yellow metals (brass and copper).  My question is are there yellow metals in the my machine?  Also has anyone ran the GL-5 in their cutter and experienced corrosion problems?

80W GL-4 and GL-5