Steve replied:

I've got mine booked in for the operation next week. 6cyl 100 series. I get around 18L/100ks at the moment and you can expect about 15% increase in fuel usage. But you'll pay less than half for your 100 kms in lpg with fuel at around 112c on average and lpg 42c. btw you'll only get 90/92 litres out of your tanks, not 100 litres.