new gas injections sytems are very good nowadays. these systems are dear (4-5k) but if you do a lot of klms it may pay for itself.
be mindfull that if your talking about the 4.0ltr v6 the valves will need adjustment more often than just straight petrol. the valve adjustment is done by replacing the valve bucket assy at approx $30 each (24 of them) just like the 1kd diesel.
i looked at one 2 weeks ago with a prins vapour injection system on it , it had 160k klms on the clock and i would say the valves had not been looked at , it had a slight flutter at idle , may be signs of an adjustment needed. i put $20 of lpg in on the way home and drove it for a week end doing approx 200klms and it still had some left - so it is easier on the hip pocket.
so finally , yes the longevity of the motor can be diminished but way it up on klms travelled to dollars spent and if (god forbid ) you had to do a head or heads reco then there goes all that money saved in the first place.
horses for courses.
i have a v6 4ltr 1gr i bought last week. del 02/06 and its got 45000klms. the trip back from sydney it averaged 12 ltrs per 100klms and diesels are getting 9ltr/100klm. so the difference between the price of diesel to petrol was 10cents last week- approx it cost me 4 cent a klm more with the petrol.
i know towing will be a different story but if a diesel gets a tank of crap fuel - the pump,rail , injectors and clean out will cost as much or if not more than a gas system for a petrol ---