If you have a crunch now , then the Castrol will not solve it.
We use the Syntrax as a matter of course when we change the oil at 100,000km in the Sooby's. If it's crunching , it still will.
Put in redline and crunch gone. Can also get them into reverse without having to wait the recommended 3 secs.
I look at it like this
$120 every 10-15,000 km
$1500 to rebuild the box and have the same problem in 50,000 odd kms with spirited driving.
Do the maths...
Cheers
Ken