Does an air filter from a 6 cylinder XH fit the XH XR8. I have an oppurtunity to get a K&N from a mate but his is from a 6.

If the 6cyl is an xr6 then yes. If its just a normal longreach or whatever then no.
Xh xr6's with the bbm manifold have the same size airboxes as EB-AU 5lt's but all other xh 6cyl's still run the eb-ed style intake setup with the slightly smaller airbox.