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Yes, You can. But quality of the mixture will be equal to the quality of the GL4 gear oil.
vauhall gear oil gl4 NOT gl5
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.
gL4/gL5 sae 75w90 for tranzmision or can use 80w90 for diffs
80w90 GL3 or GL4 gear oil.DO NOT USE GL5 Gear oil as this will eat away at the brass syncronizers in the transmission.
80-90 wt GL-4 gear lube. It needs to be a GL4 rated oil, not GL5 rated.
GL4, or if GL5 must also say on the bottle safe for syncromesh or manual transmissions. GL4 is for tranny GL5 is for axles. this is in the operators guide in the glove box.
GL5 75W90 or 80W90 gear oil.
Actually almost every single VW since the mid 80s has needed GL4 75-90 its synthetic, which VW always needs. and always GL4 not GL5, if u put the 5 in there, there are too many pressure additives and it will cause all the soft metals in your trans to dissolve.
You probably have messed up your synchros. Have you recently changed the gearbox oil? If so, did you use RedlineMTL or Redline MTL90 or GL5 as it says in the manual? GL4 (regular transmission oil) has detergents that eat up the synchro plates, you might want to go dump your oil and put fresh RedlineMTL in there (needs 2.1qts).
75w-90 GL5 gear oil
Owners Book reads: 75w-90, GL4 According with most MIata owners, best is the Motorcraft Full Synthetic Manual Transmission Fluid, it is GL4, it has additional FM (friction modifiers) to aid in synchronizer engagement, it has ford's lifetime fill rating and it is both expensive and German Second place is any GL4 75w-90 oil. it will not have the FM in it, but it will shift better than any GL5. third place is a GL5 75w-90, most common is the M1 synth 75w-90.