Thinner fully synthetic 80W/85 GL4 gearbox oil has a higher VI (viscosity index) than the OEM product, and thus performs a lot better in temperature extremes.
In Europe we could use the ROWE product, in the states probably Redline.

Anyhow and most important, let the synchro rings take the time they need, and never ever force the gears in. If forced, the synchro rings will wear out very much faster!