OK. And yes I think it is important to use gasoline too for the greasing purpose as LPG is more "dry" and cleaner than gasoline. And because of that running more hot. So the engines need to use gasoline also. Maybe 25% of the driving is a good rule as you say. Also the service is important, changing filters for example. I don't think Thai drivers are so good at that part. So better not buy a car with LPG from a Thai owner.
Then there is the brand of the LPG equipment. I would avoid Indian brands for example. There are brands from Nederland, Italy and Hungary that are better. I think that Prinse brand from Nederland is very good. Maybe there are others as well?
About hardened valve seats, how to find out what car model has that when buying a used gadoline car? Is there a way to find out?
Maybe the garages that are mounting the LPG equipment knows that so need to talk to them before buying a car?