[Simonas]

Over here LPG is a common conversion that pays off. Our gasoline/petrol is more expensive though.

If an engine is designed for LPG, it can outlive a gasoline/petrol engine. The propane/butane mix is a vapor when it enters the engine, and vapors don't really wash the oil off the cylinder walls.

If you were running an older car that was back when cars had distributors, the conversion would be relatively simple to make the car run optimumly on LPG. You'd want a cam that is for torque, raw torque, not high rpms. If I remember correctly, the big deal was with the "lobe separation", not the profile of the actual lobes. If I remember correctly, you'd also want colder spark plugs, and the ignition timing should be advanced somewhat because LPG has a higher octane rating. Ideally, if running straight propane, the compression ratio should be something like 12:1, though it should be somewhat lower if running propane/butane (LPG that most filling stations have).

$4000 is really expensive for LPG. Around here a high grade LPG system would cost aout $1400 AUD or so. We don't use CNG except for on busses. I could get a fully approved LPG system for a carburetored engine (without any lambda sensor) installed for about $400 USD with everything. Tank, reducer, hoses, wiring, valves, fill up tube, etc.

As for more wear on an engine, that is possible if using gas on a gasoline/petrol engine that does not have hardened valve seats i.e. was designed for leaded gas.

I don't think a new car is intended to run on leaded petrol...