Sam and Dave (sounds like a singing duo from the 1950’s) over at Rancho recommend LAT 80-90 wt gear oil. They included it with my rebuilt/upgraded transaxle so I used it for half a season and found that it lets the gearbox shift really easily but I found it to be a little noisy, especially in 3rd and 4th gear, so I drained it out and filled it with Valvoline Hypoid Gear Oil 80-90 wt and that quieted things down nicely, but it’s a tad harder to shift. Not a big deal, as I’m never in a hurry to “speed shift these days.