The Financial Times Deutschland, citing a complaint by the UEIL union of lubricating-oil companies, said VW granted two companies, including BP Plc subsidiary Castro, the sole right to provide lubricating oil for all VW and Audi models of a certain year's make.
"The Brussels officials are currently examining the complaint of the association," European Commission spokesman Michael Tscherny told the paper, adding that proceedings would be launched if suspicion of a cartel violation was found.
The complaint concerns VW's sales network in Germany, France, Belgium and the Netherlands, the paper said.
Volkswagen declined to comment.
Earlier this year, the EU fined VW 61 million marks ($28.52 million) for preventing Volkswagen dealers from offering substantial discounts on new Passat models.
That case followed a similar penalty in 1998, when the European Commission imposed a 200 million mark ($93.7 million) fine on VW for preventing the re-import to Germany of lower priced VW cars from Italy.