Just purchased a used high hour Hitachi EX220LC-3 Excavator. I have changed all the fluids except for the final drive motors. The factory Hitachi manual specs out a GL4 gear lube. I cannot find this fluid anywhere in bulk. I have talked to guys that are running GL5 but no one from the lubes tech lines will recommend using a GL5 due to the different Add Pack. Any suggestions would be appreciated.