In environmental terms the burning of LPG produces less CO2, virtually no CO or SO2. No harmful compounds of benzine or lead result form combustion simply because they are not added to enhance the octane of the fuel. Also note that when set against Diesel, LPG combustion produces NO particulates or ultra-fine particles.

It's not just the environment that benefits (as if that were not enough). Your engine will not suffer the attack of acidic combustion products increasing internal wear and erosion. Engine oil deteriorates at a far slower rate as no acids and almost no carbon deposits reduce its effectiveness. Less oil sludge and blockage is a large gain.