Hey,
I use subaru upper engine cleanear every service, got HM headers, 2.5" redback cat back, with Lukey rear muffler.... On LPG (std converter type) i get 500ks normal suburban driving, and aronud 600 when i go to uni alot (freeway ks 70ks each way) - On fuel i get around the 600 mark suburban driving. I have also had my Oxygen sensors changed when i put the gas system in and i usually change at about 2000-2400 depending on situation (not to say i dont go up to 4000 when i need to overtake etc)... just normal driving.