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The primary difference between GL-4 and GL-5 lies in their intended applications. Typically GL-4 fluids are designed for use in some synchromesh manual transmissions, while GL-5 fluids are formulated for both manual and hypoid gear applications.GL-4 and GL-5 are specifications set by the American Petroleum Institute (API) that determine the performance characteristics of the gear oil. Also unlike engine oils, the API specifications for gear oils are not backwards compatible with each other. …