AFTER years of filling their cars largely with the premium grade of petrol - often unnecessarily - motorists are switching to cheaper grades, fuelled largely by rising pump prices.

Latest statistics from the Ministry of Trade and Industry show that sales of the 98-octane "premium" petrol were outstripped by a "regular" grade of fuel.

The oil majors here sold 2.511 million barrels of 98-octane petrol, compared to 2.827 million barrels of 95-octane fuel last year.

Sales of the third option, 92-octane petrol, came to 0.453 million barrels last year.

Ten years ago, 98-octane petrol outsold the combined sales of 92- and 95-octane fuels by nearly five times. Even when prices started to climb appreciably from 2003, motorists bought twice as much 98-octane fuel as the other grades combined.

"Most people equate octane to power, but that is incorrect,'' said oil industry consultant Ong Eng Tong, explaining that octane is really a measure of the fuel's ability to withstand "knocking", or premature ignition.

Only high-performance vehicles such as turbo-charged vehicles and sports cars require 98-octane fuel, which Mr Ong reckoned could have led to the misconception that octane equates to power.

As pump prices continued to rise unabated, the gap between sales of 98-octane and lower-octane fuels closed. And last year, the latter - which can cost up to 27 cents less per litre - overtook 98-octane fuel convincingly.

For someone who uses 100 litres of petrol a month - the national average - the savings from using lower-octane fuel total up to over $320 a year.

The oil industry has noticed the switch. A spokesman for ExxonMobil, the largest retailer here, said: "Over the last two years in Singapore, we have seen a shift of more than 10 per cent from 98-octane to 95-octane.''

A spokesman for Shell added: "We recognise that a high oil-price environment does affect our customers' choice in main-grade fuels.''

Aircraft sales engineer Ng Tzong Sheng, 30, who drives a Skoda Octavia, said he switched from 98 to 95 "when prices started climbing from $1.20 to $1.40 a litre" about seven years ago.

Medical doctor Au Kah Kay, 45, who has been driving Continental cars, said cost was not the reason why he is using 95- instead of 98-octane.

"The technical specifications of these cars indicate that they are able to run on 95-octane petrol,'' Dr Au said. "So there is really no point in using a higher-octane rating because there will be no additional power to the car."

Motoring experts believe less than 10 per cent of the cars on the road need 98octane petrol, and suggest that motorists check the owner's manual to see what grade of fuel is recommended by the manufacturer.

Will running your car on a lower-grade fuel damage the engine if a premium petrol is recommended?

Mr Melvin Goh, managing director of Lamborghini Singapore, said the Italian sports cars are equipped with sensors, which can retard the engine timing to take lower-octane fuels.

"But they won't be able to operate at the peak," he said.

This article was first published in The Straits Times on Mar 4, 2008.