I have 2013 mazdaspeed 3 with about 92k miles and today as I was driving down the highway I was doing maybe between 60-70mph and tried shifting to 5th and it just sounded like it grinding so bad and wouldn’t go in. So I went to 6th and tried downshifting to 5th to switch lanes and it just kept grinding pretty bad. I lowered the speed and then it would go in normal but then when I was speeding in 4th gear to switch to 5th it did it again. I noticed that it was only doing this in higher speeds/higher rpms. My question would be is would changing the tranny fluid help at all? Or would my synchro for that gear going out or my clutch/flywheel going out?