Hallo,
Watch here (in Italy)
Google: lexus rx 400 h gpl klaus youtube
I have a 2004 RX 300. On 125.000 km converted to LPG with an 83 liter tank. Now on 203.000. No problems. Averages 1 liter in 6,5 km's.
It's a Dutch Vialle system. Costs: +/- 2400 euro. (No valve lube!, no problem, because I don't stress the engine a lot)
RX400h should be possible. Ask the Italian.
pjm