I'm wondering what could be reasons for a car's decrease in fuel efficiency?

Before, my car (1997 Nissan Sunny/Sentra), runs an average of 350-360 kms from full tank (around 33 liters) to almost empty.

Now, it's only running up to 320-330 kms at full tank. I noticed the decrease after I went for the regular oil/ oil filter change.

Any ideas?