lpg runs hotter than petrol because it is introduced into the engine as a gas rather than a squirt of liquid as in petrol, lpg is around 85% of the calorific value of petrol so cannot produce more heat, the heat gain is from the lack of cooling from the liquid petrol.
this can be countered, in theory, by installing an uprated oil cooler system although i don't think one exists as such. for most conditions keeping the current system in top condition will suffice.
also dont forget that p38 engines suffer whether on petrol or gas, i think if its going to go then its probably not been looked after very well in the first place.
i have seen a vast improvement in the running of my car just by making it airtight and putting new plugs in, for the sake of a few quid its not worth risking!!!
most people report that the car runs just above vertical on the temp. guage. mine does the same on petrol or gas so i can see a problem developing because of gas in this respect!!