Can it be used? I used to for my WRX and still have a new bottle unopened and wondering if anyone used it or is it not recommended for these cars?

My gear change from neutral to first and first to second is a bit notchy in the past couple of days especially in the morning. The first change from first to second feels a bit sticky. I just did APR Stage 2+ last week and took it to the mountains. I never dumped the clutch (ever) and never launched the car but under heavy throttle, i was getting this flutter sound and i think it was the clutch slipping then this morning while cold and doing 100kph and on 4th, i gave it a bit and the rpm suddenly jumped so I think my clutch is slipping. Would that cause the notchy gear change also?

thanks in advance