My rule with gear oil goes as follows; Every 2 years or whenever water gets into it. Changing mine in 2 weeks time, replacing the cover with a new one and filling it with 80W90 Valvoline High-Performance conventional.

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2010 Ranger XL 2.3L 5 Spd 3.73:1 Bone Stock
2013 Scion FR-S 2.0L/H-4, 6spd, 4.11:1 Fast Enough Stock