UK Renault Parts

In my car is a 2.2L 8V Renault engine, which to be honest has seen better days.
Does anyone know if;

  1. The engine can improveon its performance easily?
  2. If the 2.0 12V head will fit the 2.2 block without clearance issues?
  3. What the cause of my oil pressure problem could be?
3 - is probably the vaguest of all of the above, but i will explain;

Oil pressure on cold start is between 4 and 5 bar at tickover, and slowly frops off to around 0.8 once u to temperature. As the engine gets warmer, the pressure drops off to almost 0 at tickover, and i need to rev it to get it up (ooo errr).

Is this because as the oil warms up it becomes thinner and loses viscosity and as such reduces in pressure, or could this be a different problem? Oil is new, as is the filter. Maybe slightly thicker oil would be better?

Ive considered the oil pump, strainer on the pickup, and the intermediate shaft, but i dont think that the oil pressure would drop gradually if it were these, but stay low from the start??

Your thoughts would be appreciated.