This paper presents an aggregate oil degradation indicator, developed by the authors, as a surrogate condition indicator for changes in the physicochemical parameters of engine oil during its operation. In the future, this indicator will be used to develop a novel method for determining the Remaining Useful Life (RUL) of engine oil used in ultralight aircraft operation. The indicator was developed based on laboratory analysis data of engine oil samples collected at regular intervals from ultralight gyrocopters operated at a flight training school. To determine the degree of oil degradation, an indicator incorporating 5 out of 35 analyzed parameters was proposed. The parameters were selected to simultaneously satisfy the following conditions: the parameter is determined during standard laboratory testing, and it exhibits a monotonic variation correlated with an increase in total flight hours or flight hours since the last oil change. The following parameters were selected: iron content, aluminum content, copper content, sulfation, and oxidation. The paper demonstrates that the proposed parameters exhibit a noticeable correlation with the oil operating time between replacements—observed over a period of 11 consecutive oil changes for two aircraft—which is stronger than that of similar parameters derived from the literature review.