The rear diff fluid only really matters if you own an STi. The WRX and older RS had sealed viscous units, you could throw anything in there that can legally be called "gear oil" with no ill effects provided it's GL-5. Same with open diffs. Because of that I don't think I ever bothered with rear diff oil on the RS. My Miata has standard Redline 75w90NS (no friction modifier) for the Torsen. Or maybe it's 75w90... whichever one is slipperier... I can never keep Redline's gear oils straight.
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FWD is the new AWD
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