So I now have 10k miles on my car. Right about 500 miles ago on a random normal shift into 5th it grinded pretty loudly. This was a normal shift at about 2500rpms into 5th from 4th. Ever since then, when the tranny is warmed up, it now grinds. The only way I'm able to prevent it is to shift so slowly, that the rpms have fallen to idle before the synchro will allow me to gently guide the shifter into 5th without grinding. Anything above that and it grinds. I did notice that while cold I can shift into 5th without issue.
I figured the Extra-S transmission fluid I put in for some reason isn't handling the heat anymore and decided to replace it with Motul 75w90 gear oil which was recommended on a lot of forums.
Unfortunately that did nothing. If anything, shifting into 5th is even rougher now with more grinding. All other shifts into 1-4 are perfectly smooth.
When I replaced the fluid, I noticed on the drain plug (which is magnetized) some black slimy goop stuff. The tranny fluid was light brown but this goop was solid black. I wiped the drain plug clean but i'm wondering what this is. Synchro material or maybe fine metal particulates that settled on the plug.
Either way it concerns me.
My next step is the dealer. I'm just not looking forward to them either rejecting the warranty claim or actually replacing the transmission.