Thats sad , probaly you got a small hair- fracture somewhere, maybe you can repair it , there is some stuff around that resists heat , but probaly can't cope with the pressure in your engine.
Maybe you can ad a viscosity increaser to your engine oil , maybe when the viscosity increases , the oil can't leve the engine.
I don' t reconmend this , removing the engine and at N2 pressure( .5bar) to it , search the leak (use leakfinder aerosol ) and replace the leaking part sounds obvious but isn't easy.
I don't understand why it takes so long for yur garage to find the leak , trying to make it leak??
replacing the complete engine is cheaper I think , here in Belgium you pay around 250-500€ for an engine of a total loss car (prefered T- loss behind). If you count the hours spent (I pressume your garage don't work for free).In Finland there are fewer cars around so prices will be more expencieve I think??
One day the sun will shine again