Sun Oil Inc. would like to find up to three oil-producing nations as partners in running its U.S. refining and marketing business, the firm said. The unit is its most lucrative, accounting for 83.5% of its $9.48 billion in 1987 revenue. Sun markets its refined products under the Sunoco and DX brands in 6,606 service stations across North America. A spokesman declined to identify the countries that Sun has talked with, but Sun Chairman Robert McClements Jr. confirmed in July that Nigeria was among the potential partners. Less than 4% of U.S. refining capacity--totaling nearly 16 million barrels a day--is linked to foreign oil producers, said Petroleum Analysis Ltd. analyst Dillard Spriggs.