Hello Eugen,
Sounds as if your mechanic has made a reasonable assumption that it is the oil pressure sensor is at fault rather than the engine actually having low oil pressure, but an assumption it is.
Even if the new sensor solves the problem, you have no idea of what the running oil pressure is - it may only be marginally higher than that needed to put the light out.
Proper action:- remove the sensor and replace it with a properly connected oil pressure gauge - which the garage should have. Take the car for a 10 mile run to get the oil up to operating temperature, and check the oil pressure at idle and above say 2000 rpm. You will then know if the pressure is acceptable.
Motul oil? Just a personal opinion, and without full knowledge, but I’ve always taken this oil to be ‘budget’, and generally available from specific internet suppliers. I’ve always used Quantum oil (was/is recommended by VAG) since it is only marginally more expensive. I use the correct grade in their Longlife variety, but still change it once a year on a low mileage schedule.
Kind regards,
Gareth.