Hi Mark.
Putting a cone filter will raise your fuel consumption. It`s because your engine will be `breathing` hot air from inside your engine bay.
If you mainly do `open road` driving a cold air feed would cure this. But in town, its a big no-no..
The other problem is the great sound. It makes you want to put your foot down at every opportunity (more fuel again)
For a performance boost, you should fit a K&N panel filter in your original airbox.
Neither filter will damage the engine.
As for noisy fifth gear. sounds bad.
Fifth is its own little gearbox on the end of the main box. The bearing sounds tired if its just one gear and under stress.
Very expensive fix, especially when there are plenty of breakers out there.
A replacement box is easy to find, Just make sure its from the same size engine, for the correct ratios.
