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GL3/GL5 Gear Oil Ratings
GL5 gear oils can attack the yellow metals found in manual transmissions. GL5 gear oils are recommended for differentials. GL3 is safe for the yellow metals, and should be used in transmissions. GL3 fluids should not be used in the differentials.
With that being said, GL3 is an all but obsolete specification at this point. While you can find some GL3 fluids still, they are pretty rare. Which brings up GL4 fluid. To keep this simple a GL4 does not have the level of additives that GL5 does. This is what most people choose in absence of GL3 fluids for transmissions. Note this does not mean fluids that say GL4 / GL5 are acceptable. An example of a popular replacement fluid for the AX5 / AX15 is Redline's MT90 GL4 75w90.
Related Writeups: | Manual Transmission Fluid Change | Changing Differential Fluid |

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