A friend of mine has a Cayenne 4.5S and, apart from some expensive irritations, it has been reliable. More in car-by-car breakdown at hjuk. I don't recommend LPG conversions not approved by manufacturers, particularly on any engine with variable valve timing designed for the burn characteristics of petrol, not LPG. An old Range Rover V8 was a very simple pushrod engine, dating back to the 1960s Buick Skylark. Jeep 4.0 straight sixes are also pushrod, simple and easily adapted to running on LPG. I’d better add that Beside which, predictably, the UK government is increasing the tax and reducing the benefits of running on LPG, just like it did the last time round.