I happened to have Redline's web site open. Here what they say about the products we should be using:
75W90 Gear Oil - the preferred product in nearly all car and light truck differentials, both conventional and limited-slip. 75W90 is preferred for most racing applications. It contains friction modifier which is recommended for limited-slip units. 75W90 Gear Oil can be used in many transmissions and transaxles; however, other Red Line lubricants have better frictional properties for rapid synchronization. Exceeds API GL-5.
MT 90 - Manual Transmission/Transaxle Lubricant. A 75W90 GL-4 gear oil which is designed for manual transmissions which require 75W90, or 90 GL-4 gear oils or motor oils. Uses the same chemistry as MTL , but is a 90 WT in order to satisfy certain manufacturers which recommend SAE 90 GL-4 gear oils.
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