Heat vegetable oil in a pot and place biscuits in heated oil. Oil should cover biscuits. Cook until golden, about 1 minute on each side. Remove from pot and roll in sugar. Allow to cool. Using a knife, make a small hole in the side of each doughnut and fill with your choice of filling (custard, jelly, marmalade, etc.). You can use a ziploc bag for the filling, just cut off one of the bottom tips and it will allow you to pipe in the filling. If you would prefer unfilled doughnuts, prior to cooking, you can make a hole in the center of each biscuit and they will come out looking like normal doughnuts.