I am going to bring my 05' Mazda 3s in to get the manual gear oil replaced, since I'm around 30K. I asked the dealership and they replace the oil with their usual stuff, not synthetic (but they could). Is it really worth switching to synthetic gear oil? Is there proof or experience that a difference can be felt?

 

redline MTX 90 or GM synchromesh. I've heard good things about the Royal Purple gear oil but I've not used it myself.

 

I don't know if you can feel the difference, especially if your tranny's shifting smoothly, but for something that only gets changed every 30k miles, IMO it's worth it to spend a couple extra bucks on the good stuff. Personally, I like NEO or Syncromesh.