The study aimed to determine the theoretical air–fuel ratio (AFR) for ternary fuel mixtures composed of gasoline, ethanol (E5 and E10), and lubricating oils (mineral, semi-synthetic, and synthetic) used in two-stroke spark-ignition engines. The background of this work arises from the widespread introduction of ethanol-containing fuels in the European Union and the necessity to calibrate two-stroke engines operating on mixtures of gasoline and oil. The research was conducted using stoichiometric combustion equations for each component, considering their molecular formulas, molar masses, and densities. Volumetric fuel-to-oil ratios of 1:25, 1:50, and 1:100 were converted into mass proportions to determine the composite AFR for each blend. The obtained stoichiometric AFR values were: for E5 fuels — 14.42 (mineral, 1:25), 14.41 (mineral, 1:50), 14.40 (mineral, 1:100), 14.40 (semi-synthetic, 1:25–1:100), and 14.36–14.38 (synthetic, 1:25–1:100); and for E10 fuels — 14.13–14.10 (mineral), 14.11–14.10 (semi-synthetic), and 14.07–14.08 (synthetic). The influence of oil type and fuel-to-oil ratio was negligible, with AFR variations not exceeding 0.06 and 0.03, respectively. In contrast, increasing ethanol content from E5 to E10 decreased AFR by approximately 0.30. Corresponding correction coefficients for analyzers calibrated to pure gasoline (AFR = 14.7) were 0.98 for E5 and 0.96 for E10. The results provide a quantitative reference for accurate AFR calibration, carburetor adjustment, and lambda correction in two-stroke engines, supporting the adaptation of diagnostic systems to modern ethanol-blended fuels.