I am doing a comparison with baked to determine the amount of added fat.
I sold deep fry oil for a few years. According to company and industry studies, it could add 10%, but up to 1/4 of the weight if using the wrong oil or wrong temperature. Have you eaten a doughnut and it has the fatty taste and “bad mouth feel”- when the fat coats the tongue? They aren’t using the right oil (which melts at a lower temperature), and chances are frying below 350°F or not waiting for the temperature to go up again after frying a previous batch. A properly fried doughnut absorbs less oil and saves the restaurant or bakery money because they can buy less oil.
Mesure the oil before frying the donuts, and measure the oil after you finished frying the donuts and divide the difference between the number of donuts.
Let's say, for (simplified) example you want to fry 12 donuts have 4 liters of oil at the beginning, and you have 3.8 liters at the end.
So there's about 200 milliliters that has been absorbed by the 12 donuts, so that about 16 milliliters per donuts.
You can also expand the experience by varying the oil temperature, the donut batter recipe... and see if you can find a good median approximation.