No, it is not normal and get the dealer involved... be adamant about it... say it started only a month ago and things are getting worse. They probably will do a leakdown test... compression test to determine if engine is out of spec. I am assuming your dad always had the dealer service the car since it is under extended warranty... so your dad should have a good relationship with them...
Tell them they are the only one servicing the car and has record for 6 years.
Do not wait anymore. Get it done.