Earlier, I created a mathematical model of the gear unit, in which the value of the tooth friction was determined during precalculation and then entered into the model as a constant parameter. In this paper, I investigate how much viscosity oil is required at different operating points for a single-stage gear unit with plastic gears to function properly and how this affects the tooth friction and, hence, the loss of energy in the gear unit. Once the investigations are complete, the tooth friction is determined using an automated MATLAB function, which allows the tooth friction to be re-determined at each operating point. This allows for a more accurate strength dimensioning of the plastic gears in the gear unit and a more accurate model describing the gear unit’s behaviour.