Please excuse me for using English, it is because I don't speak Dutch

I need some support for S40 1.8 122hp from year 2003.
The car is with a LPG system - Vialle LPI with liquid Injection and I mostly drive it on LPG.
I decided to seek your support as this model of Volvo and Vialle are Dutch products and I hope
here are many people that have dealt issues with them.
What are the suggested type of Spark Plugs by Volvo for this car?
Is there a type of plugs that you prefer for this engine driven with such LPG?
I drive the car around 40000 km per year mostly on LPG and for me the most important issue
is the fuel consumption and the engine performance.
Thank you