gearbox man said :
“The SAE90 will do the job, however, filling with a full synthetic 75W/90 will give a better stability when hot.”

Mike rainbird on 2wd diff oil :
Also, I use a top quality fully synthetic gear oil such` as Mobil SHC, which is rated to a lot higher temps than I ever saw...

Jim Green :
Use Mobil Mobilube 1 SHC SAE 75W-90 (Extra High Performance Synthetic Lubricant for Gears and Axles)..... Mike Rainbird can supply this overnight. It's the best protection you can get for the diff, especially bearing in mind track use.