The ideal temperature is around Heat the oil to 165-175°C / 330-350°F. The dough contains eggs and butter which causes them to brown quickly. I prefer a slightly lower temperature, to ensure the interior cooks before the exterior gets too dark. If the oil is too hot, you'll end up with raw dough inside but also if the oil is below 165°C/3305°F, you risk the donuts absorbing oil and becoming greasy. It's best to invest in a candy or frying thermometer to maintain the right temperature.