You can use:

-Valvoline 80W90 gear oil ($2.8 per quart),

-Valvoline 80W90 Durablend semi-synthetic gear oil ($4.4 per quart)

-Castrol 75W90 gear oil ($3.2 per quart)

-Mobil 1 75W90 synthetic gear oil ($6.99 per quart)


Castrol also has a 75W90 synthetic gear oil whose specs beat those of Mobil 1 75W90 synthetic gear oil, but it's hard to find and costs about $12 per quart

I prefer Valvoline because it's cheap and good brand.

Eric