When a Brooklyn natural gas company compares gas and heating oil prices in its advertising, it must warn customers that natural gas prices may go up because of gas price decontrol or other events, the New York Public Service commission has ordered.

Since late October when the order was handed down, Brooklyn Union Gas Co. has noted in its advertising that the price of natural gas, or oil, may fluctuate due to national or international events, said Bob Loftus, a spokesman for the utility.

The New York Oil Heating Association, a group of fuel oil dealers in New York City and it suburbs, had intervened in a rate increase case. The association argued that the gas company had an obligation to disclose the potential cost-increasing impact of decontrol of natural gas prices in any ads encouraging customers to switch to gas. Prices for some categoires of natural gas are scheduled to be decontrolled in 1985.

According to the gas company, which said it had approximately 10,000 conversions in the last year, the intervention was sour grapes. Even though heating oil prices have been held down recently by a combination of warm weather and reduced demand, gas is still selling at "a 20 percent differential," according to Loftus. "It's not only the price advantage, as much as the cleanliness and reliability of natural gas" that has lured customers away from fuel oil dealers, he said.