Hydraulic. If you could be bothered, pull them out and clean them, or simply replace them with new ones from Nissan. The VG30 ones are the same I believe (but check with nissan before taking that as gospel).
The pinhole in the centre of the cam get blocked, and they stop pumping up.
Ticking means the valves won't be opening as far as they should. And the cam surface will take a bit of a beating if left that way for a long time.

Maybe try a thinner oil? Depends what you are running in the engine right now.