Australia's biggest oil refiner also said it wouldn't mind if the Federal Government's FuelWatch plan did not come to fruition because it had a neutral position on the proposal.

The Government wants to introduce the scheme, which requires petrol retailers to advertise prices 24 hours in advance, as a response to rising cost pressures on voters.

Caltex chief executive Des King has told a TV interviewer, ''I think the public is starting to focus a lot more on the crude oil price and that it is a major constituent of the petrol price.

''There's also more focus on the fact that the tax on petrol is over 50c a litre.'' Mr King said Caltex made a profit on petrol 1.5c a litre last year.

Light, sweet crude for October delivery plunged by $US6.59, or 5.43per cent, to settle at $US114.59 a barrel on the New York Mercantile Exchange on Friday.

It was crude's largest single-day price drop, calculated by percentage, since December 27, 2004, when prices dropped by 6.47 per cent.

Crude oil prices have now tumbled significantly from their July high of $US147 but are still almost twice as expensive as two years ago.

FuelWatch appears dead, with Family First senator Steve Fielding joining Independent Nick Xenophon last week in opposing the scheme as it stands now.

Mr King said, ''We don't like to have more regulation in our industry: there's a lot already.

''But overall the impact on our business, we feel, would be quite neutral whether it was in or out.''

Mr King said that under FuelWatch people who filled up every Tuesday would be losers because the price cycle would have to be extended to 10 to 14 days from seven days.

People who filled their tank only when it was empty would probably benefit.

Caltex reported last week a 4 per cent fall in net profit, including inventory gains, to $354 million for the six months to June 30.

Caltex said a drop in refiner margins, from 8.32c to 7.02c a litre, and shutdowns of its two refineries, were significant contributors to the company's interim earnings fall.

Caltex's refinery production in the first half was lower because of a planned maintenance shutdown at its Kurnell refinery in Sydney and unplanned shutdowns at Kurnell and the Lytton refinery in Brisbane. AAP