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Yes, You can. But quality of the mixture will be equal to the quality of the GL4 gear oil.
GL5 75W90 or 80W90 gear oil.
vauhall gear oil gl4 NOT gl5
75w-90 GL5 gear oil
Recommended GL5 SAE 80W90 hypoid gear oil. Fill to bottom of fill hole.
80w90 GL3 or GL4 gear oil.DO NOT USE GL5 Gear oil as this will eat away at the brass syncronizers in the transmission.
gL4/gL5 sae 75w90 for tranzmision or can use 80w90 for diffs
BMW recommends ONLY using BMW oil that can be purchased at their dealerships for their motorcycles, including the 1987 K75. For the transmission, you can use 1 Quart Mobil-1 GL5-75/90 or Castrol SAF-XO GL5-75/90 Synthetic Gear Oil.
my book says 80w90 gear oil. if it a limtied slip you gl5 with 80w90 wt
1986 to 1996 Manual Transmission and Transfer case use API GL4 or GL4 and GL5 80W90 or 75W90. Differentials use API GL-5 SAE 75W-90 or 80W-90 lubricant. Do not put a GL5 gear oil in the transmission unless it says GL4 and GL5. GL5 will attack the syncros.
there is a drain (on the bottom) and fill plug (toward the top) for the front differential. 1.6 quarts of 75W90 GL-4 or GL5 gear oil.
80-90 wt GL-4 gear lube. It needs to be a GL4 rated oil, not GL5 rated.