6 speed conversion on 2008

[rprice]

My 5 speed died in my 2008, so I decided to replace it with a 6 speed with 87k. Everything installed without issue and the car drives fine, except that I cannot keep it in 6th gear. As soon as I accelerate it pop itself into neutral. I am assuming that I got a trans with possibly a bad synchro, but figured I would see if anyone experienced this or if anyone knows of something else that may cause this

[SeattleJeremy]

What gear oil are you using, does it have enough?
Are the shiftier bushings in good shape, and the cable isn't bunching up?
Is there any grinding?