A new air filter always helps. Fuel injector cleaner is a good thing to help gas mileage. Tires are another thing as well. Your tires need to be at proper inflation in order for your gas mileage to do better as well. In terms of the yellow spillage there probably is a leak somewhere and the only way to find out is to get underneath the car and look. It could be the tank, hoses, connections, etc...

Good Luck!