and you have a diesel car, take it with you to filling station, and put half in the tank before you fill with diesel.
Next time you fill put the rest in.
We had a very smoky leon and used to put in 5 litres of petrol along with 50 litres of diesel the day before MOT. No smoke for mot.
New airflow meter and clean the EGR valve eventually cured it.
This is lower than the permissible petrol content for very cold weather according to VW.
Myke