A class action lawsuit has been filed against the vehicle manufacturer for allegedly changing maintenance specifications which resulted in higher oil change costs. Owners of 1997-2004 Audi A4s and 1998-2004 Volkswagen Passats were notified by letter that they would need to use more expensive synthetic oil, over less expensive petroleum-based oil. According to the suit, the cost of oil changes on these vehicles more than doubled. Furthermore, even if owners had complied with the new requirements specified in the letter, engines could fail due to the use of the previously required oil filter. Vehicle warranties are honored only for owners who can prove they have maintained their vehicles on schedule.