I know Hondas better than just about anything, so they get their own section here. As for fluid, I know that the Honda manual says you can used 5W30 or 10W30 motor oil in their transmissions. I do not recommend this. In fact, I recommend that you only use Honda fluid in your Honda manual transmission. There is one exception; I have found one other fluid that works well and that's GM's Syncromesh fluid. I've actually had luck fixing stubborn synchronizers using this fluid, so it's something to be considered. In fact, I know some GM guys who call it "transmission rebuild in a bottle." If you have a shifting problem with your Honda manual, check the clutch hydraulics for leaks, particularly the clutch master. These have been known to leak, and when they do they can cause shifting problems, usually grinding into gears.
In addition to checking the hydraulics, check the shift linkage for worn bushings. This is also a common problem with Hondas. This linkage is often located above an exhaust shield. You might have to drop that down to do a proper inspection. If the bushings are worn, replace the linkage; it's much easier than trying to mess with new bushings.
Lastly, Honda third-gear synchronizers are known for faults. If you only have a shifting issue going into third gear, you might need to replace the third-gear synchronizer.
If you put a performance clutch in your Honda and you have a hydraulic clutch, be prepared to replace the clutch hydraulics often. Performance clutches have pretty stiff pressure plates and the already-weak Honda clutch hydraulics don't hold up well under the extra load.
Here's a video on changing Honda manual transmission fluid.