New European Union regulations will set a limit on the amount of carbon dioxide that the entire fleet of cars by a particular carmaker is allowed to produce on average. The amount is set at 130 grammes per kilometre by 2015, and 95 grammes per kilometre by 2020.
But carmakers can dodge the limits by building a few electric cars, which will bring down the average emissions for the whole fleet.
In the letter, dated July 6 and quoted in the Süddeutsche Zeitung on Thursday, Oettinger told Winterkorn that "the discussion about our CO2 policy for cars after 2020 will be completely open."
The letter was a reply to a request from Winterkorn to the commissioner, dated July 5, asking him to make sure that VW would not be put at a disadvantage by the new restrictions, which were released on July 11.
"Oettinger has contributed to watering down the climate requirements," Greenpeace climate expert Franziska Achterberg told the paper.
The European Commission is also doing what it can to protect the auto industry in other ways. Industry Commissioner Antonio Tajani recently called for Europe's industry to be the bloc's main priority. A joint op-ed written by Oettinger and Tajani in the Frankfurter Allgemeine Zeitung said that the car industry needs to be protected from "a disproportionate burden."