Combine the flour, sugar, eggs, and milk and beat until smooth. The resulting batter should be the consistency of thin cream. Add oil to the batter and mix lightly. Grease an 8-inch crepe pan or frying pan lightly with butter and heat until the butter is quite hot but not burned. Ladle about 1/3 cup (or a bit less) of the batter into the pan and rotate the pan to spread the batter evenly. Cook the crepe until it looks firm and is lightly browned at the edges (about 1 minute) then turn the crepe over with a thin spatula or your fingers and cook the other side for about 30 seconds. Grease the pan with a bit more butter about every other crepe, or when the crepes begin to stick. Repeat until the batter is gone.