Classic Impreza turbo 4th gear crunching

[PotatoSalad]

I've got a MY00 classic UK impreza Turbo, running on stock power, 97k miles.

The 4th gear started grinding very recently while downshifting from 5th. The gearbox oil was changed 2 years ago (April 2015) to Comma 75w90 @83k miles and it's been driving perfectly until about 2 weeks ago when the crunch happened once or twice on a longer journey and went away. It started again yesterday when downshifting, usually on higher revs, it's quite smooth around 2000rpm. Double clutching or rev matching help, shifting up from 3 is smooth.

I guess it's a worn synchro. Is it worth trying a different oil or the famous coctail? Some forums suggest changing to Motul Gear 300 75w90.

Worst case scenario how many hours of garage labour should I be prepared to budget for to change the 4th gear synchro? Would it be cheaper to get a reconditioned gearbox and swap it? I'd rather keep the car running, I've had it for over 3 years now, been servicing it, done the belts, welded small rust patches on the rear arches and resprayed scratched front bumper, it's nice to drive something that's getting rare these days, especially in red.

Any help greatly appreciated.

[vxr2010]

do fluid first the redline shockproof from memory , if it's box related there was a company doing rebuilds which i think they had in stock i can't remember the name of that company , zen is one company and there is a chap called colin who works near silverstone , good luck hopefully fluid change will do it

[anonymous-user]

Don't use shockproof in a Subaru box, it uses a centre shaft oil supply system that centrifuges out the shockproof balls and can starve the bearings.

I use Fuchs (Silkolene) Titan Race, but any conventional GL5 oil should do the job if just pottering about.

Slow the gearshift slightly and you will probably find its OK for a while longer.

[vxr2010]

the reason i would do shockproof as you have little to loose , i only used a litre of it the rest a good gearbox oil and the notchyness was much better after , once the box warms up its fine

[anonymous-user]

Just don't. Redline themselves say not to use it.

[plenty]

I had a 4th gear crunch on my newage box. 50% Lightweight Shockproof and 50% Millers 75w90 solved it and 20k miles later it's running without any issues.