I would perform a full oil change with new filter and a triple ester oil such as Fuchs Titan Pro S, Motul 300v or Millers nanaodrive in a 5 40 viscosity. this is the same thickness cold and slightly thicker hot so prevents burn off at high rpm.
I would drive slowly and allow the engine to warm up gradually for the next month and only give it some beans after a through drive - say 20 mins of mixed driving.
So far this has maybe cost a 100quid but its ruled out the oil etc.
If it is still losing oil then I would investigate if your burning it or losing it.
I assume you have checked visually for leaks etc? any blue smoke?