I've had my RX-8 since novermber of 2010. And since the day I taught myself how to drive manual on it, it's been grinding the 4th gear. It doesn't seem to do this if I COMPLETELY come off the gas, as opposed to "blipping the throttle" to make shifts smoother. But sometimes it completely "hops" the synchronizer (not engaging, no resistance upon shifting). The car has 53,000 miles on it and hasnt had the gearbox replaced(to my knowledge...) so I know it's not a breaking in thing. Any thoughts on what this could be? Possibly a bad 4th synchro?

 

I've tossed one transmission out because of bad 2nd and 3rd synchros.... a previous owner tried driving with a completely nonfunctional clutch master cylinder... but, anyways, before you try rebuilding/replacing, drain the old transmission oil out and replace it with something good... I almost completely fixed my old transmission grinding issues by running the 75w90 Max Gear RP oil/25-35% Lucas Oil Stabilizer mix... the 3rd synchro grinding was completely fixed, but 2nd synchro was a total goner, so that's how i ended up with a new transmission... but yeah, try 75w90 Max Gear RP oil/25-35% Lucas Oil Stabilizer mix... if you can't find Royal Purple oils (usually sold in NAPA), go with Red Line 75w90 equivalent... Red Line oils are very good too

while at it, drain the clutch hydraulics to make sure that a weak clutch disengagement isn't contributing to the issue..
hope that helps.