You have to strain the vegetable oil first. I'm assuming you're using waste vegetable oil, since it's not cost effective to use unused oil as a fuel. You can get WVO from any restaurant that uses oil (which, be honest, describes all of them), usually for a small to no fee. I've heard of people putting it directly into their tank, but it does hurt the fuel lines, and it gels up in cold weather. You can combine vegetable oil and diesel, but you're never quite sure how much of each you have in there. Also, I heard putting vegetable oil in your tank may screw with your car's emissions test. There are separate fuel tanks you can buy specifically for vegetable oil that are easy enough to install if you know what you're doing. How it works is the second tank goes in the trunk. You start with the regular diesel in your original fuel tank, then flip to the second tank (filled with your oil) after the car's warmed up, which takes about 5 minutes depending on the car and the weather. When you're about to park the car, just switch it back to the regular diesel. This system does make up for the cost, though you need to modify the car to be able to use it. BTW, the best vegetable oil is from Japanese and Chinese restaurants. It's the cleanest, which means less filtering for you. Be sure to strain the fuel well, or else that gunk builds up in your fuel system and you'd need to replace all of it. I've heard good things from people about this. Vegetable oil gets you about 60 MPG, give or take. Good luck if you're serious about it. Just Google WVO (or waste vegetable oil) and there are several good sites about it.