Hi there,
I've got a 2000/X GS430SE which I've just had converted to run on LPG - it's fantastic really and on a run I'm getting the equivalent of 40-50mpg, which is amazing for a car that goes like stink when you want it too! There are many places where you can have LPG fitted, but I used Profess Autogas near Cardiff. They really know their stuff, use the very latest multipoint system and even provide a fully fueled LPG courtesy car for your use whilst the car is being done. When I picked mine up, they also included a software pack with lead so I can check and adjust the settings via my laptop when required. You can find Profess advertising via eBay where they have 100% feedback. Should cost less than £1200 inc VAT to convert a 6 cylinder GS. Some people mention concerns with valve wear, but this only occurs when the system is set too rich or lean, a common problem with older single-point systems. The software electronically matches the mapping of your car's ECU so reducing this risk.