Outside tanks have a habit of collecting water, and if you tilt the tank to keep the water at the low side, you run the chance that eventually the water will rust out the bottom of the tank, and then you have an oil spill problem. If you put the outlet side low, add a good-quality water separator to the line, and keep an eye on it, you are less likely to have any problems. When I had an outside oil tank, I plumbed it with two lines, one to the burner pump, and one from the burner pump back to the tank. I filtered the oil coming and going, and change filters regularly. Never had a problem with the setup as described. I also added a fuel conditioner that was supposed to eliminate the water and it must have worked because the separator rarely needed to be drained.