Mobil 1 Engine Oil
Ok, so it's not really a mod, but I have to list it. I've been putting nothing but Mobile 1, 10w30 or 5w30, into the engine since I got it at 96000k. I'm not sure which viscosity I like better yet, so I've been trying both.

Non-Synthetic Transmission Fluid
I used to have the highly-recommended-by-many Redline MTL in my transmission. Not too long after changing to this, my transmission started getting notchier. As of a week ago, my 3rd gear synchro was completely toast. *CLANK* *GRIND* became a regular sound. My mechanic said that he's now convinced that synthetic fluid doesn't lubricate the synchros properly in Mazda transmissions, causing them to fail. All fine and dandy, but it means I needed a new transmission. Oh well. Live and learn.