Hi there, wondered if anyone can help with advice on the below?

I bought my VW Passat from a franchised Toyota dealer in November 2012. As part of the agreement before picking up the car they said they would do a service on it as the service indicator was blinking on the dash cluster.

However, cynical or not, I am now wondering how much of a service they have done, as inspection of the air filter showed it was extremely dirty and well over a year old. Also, the oil filter cap is unbelievably tight and unsure whether that has been replaced recently

The car came with a complimentary one year full used Toyota warranty, which touch-wood has not had to be used yet. However as the car has a turbo and I want to look after it, I would like to do my own oil change which I have done on other cars in the past.

Would this technically (or otherwise) invalidate said warranty? I am using all genuine VAG Parts - oil filter is genuine, as is the Longlife Quantum III 507.00 spec oil. Surely any inspection if it came to it would show that all parts were genuine Volkswagen.

Thanks for any assistance. I have a long holiday journey in May to make and would feel happier knowing it had fresh oil in, and more importantly a new filter!