[CJR_STi]

My 2005 STi (320whp & 350wtq, OEM wheels & tires, suspension and drivetrain with ~ 26k miles) is making a crunching noise downshifting from 5th to 4th gear. All began at the last trackday and is getting worst, louder and more difficult to select that gear. What can it be???

Is strange because at normal driving (without heel & toe or any other technique) it’s difficult to downshift to 4th with the engine above 2500rpm. Upshifting from 3rd to 4th is as normal, without any strange noise. Although I’ve tried downshifting from 5th gear into 3rd without disengaging the clutch and then back into 4th and everything is smooth as new.

Already change the tranny fluid with Redline 75W90, but don't remember if it was lightweight or regular synthetic gear lubricant. The problem feels and sounds in the same way as before the fluid was changed.

Seems like a synchro problem, but did anyone have experienced a similar problem?

[Rootus]

The 6MT tranny is definitely sensitive to fluid choice. What you describe does sound like synchro issues, possibly caused by fluid choice. There is a lot of anecdotal evidence suggesting these trannies do not care much for Redline fluid. From my research it looks like perhaps the best choice we know of right now is Motul Gear 300. That's what I'm planning on using in mine shortly as it's getting to be time...

[klaymon]

It's a synchro issue, but as far as whether it's wear or fluid related leaves to be seen. I had the same problem with mine at 3rd gear. Mine was fluid related. I was running Motul Gear 300 and 3rd was quite a chore sometimes. I went back to Valvoline 75w-90 non-synthetic and everything is fine now.

[jarvis210]

just get the oem subaru stuff and its definitly different than valvoline 75-90 becuase valvoline is a brownish tint and subaru is clear. just got mine changed at the dealership and it shift like the day i got it off the lot.