Toyota recommend ATF in the SV21 Camry manual gearbox, the same as used to be recommended for the R380 in the Defender. I believe that is because like the R380, there is an oil pump in the gearbox.

Since a lot of people prefer Syntrans, MTF94 or some similar low viscosity manual transmission fluid in the R380, would it be safe to assume that the Camry gearbox oil pump would cope with similar oils?

The reason I ask is that third gear synchro in my 1992 Camry is quite adequate while the gearbox oil is cold. After an hour or so at highway speed, the synchro on third is completely useless. All the other gears still have effective synchro.

My understanding is that the synchro relies on friction or viscosity to actually work and that as the ATF warms up, it becomes thin enough to no longer work.

I was hoping that something like 75W-80 or better still 75W-85 might not thin out as much as the ATF and that the synchro might continue to have some effect when things warm up.

Can any of the oil gurus here see any flaws in my thinking?