The rear suspension has a inner tube air valve on the under side of the reservoir cylinder.

What pressure range is recommended/ are people setting their bikes to?

im fitting yz ones to mine not a strait fit tho bottom yoke need to be bored slightly

The 'air valve' takes very high pressure nitrogen. In the order of 200psi I think - not an item you should mess around with if you don't know what you are doing.

The MK 3 suspension is better than the MK 2, are any of the bits from the forks interchangeable?

Given that either YZ or CR springs either fit are a direct fit or can be made to fit, what can be done about the damping?

How much of an effect would using thicker oil make?

Would any of this actually help or simply make the suspension harder and less comfortable?