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If it's crunching gears when changing then something is worn, or broken. The condition may be able to be masked with different oil for a while, but wear is wear, broken parts are still broken, & in the end there is really only one way to overcome the symptoms.

We all know that these things don't heal up like a scratched finger does, so it may be wise to start looking for someone who can overhaul it, or to start finding parts sources & fix it yourself.

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Almost cheaper to find a new box than use that stuff

thanks

to answer the question 75w90 is a good safe oil to use in the tranny.

redline lightweight shockproof gear oil might help keep it alive.

I'd only use this on gearboxes that are sick otherwise castrol synthetic gear oil is the weapon of choice.

It may sound wrong
But T5 boxes in Falcons run Dextron 3

sometimes dumping all the gear oil and filling it up with a lighter weight or even just clean oil can do wonders after a bit of driving.

Using a lighter grade oil (automatic transmission fluid) has been recommended to me in the past for bad synchros in R30 Stumpy gearbox. ...I know it sounds wrong.

soory but here in portugal we call valvuline to all the transmitions anda gearboxes oil.

the problem i have is one sincro ruined.
if i try to pass fast from 3 to 2 gear i ear a crunch.

but if i reduce slowly from 3 to 2 theres no problem.

Noises is either bearing wear or gear wear. Using a thicker gear oil may help but it's not the solution. Using a thicker oil may affect the synchro operation. Making the gear changes crunch..this will be more annoying that the gear noise I suspect.

You will need to buy a workshop manual, and strip down the gearbox, and if any of the bearings are making crunchy or 'dry running' noise, then the bad bearings should be replaced. To get a gearbox service business to overhaul your gearbox will cost you 10 times as much.

Good Luck,
Chris