Under EU mandates, sustainable aviation fuel (SAF) reaches airports blended into Jet A-1, yet engine data for such commercial fuel are rare. This exploratory study compared it with fatty acid methyl ester (FAME) blends in a JJ-1400 microturbojet. Both campaigns used the same Jet A-1 reference, and all fuels contained 5% v/v turbine oil. The first tested 25% and 50% FAME at 60,000–120,000 rpm. The second tested 20–100% of a commercial Jet A-1 containing up to 50% (m/m) renewable component at 50,000–120,000 rpm. Fuel effects on thrust and exhaust gas temperature (EGT) at fixed shaft speed were estimated with an additive two-way model. The thrust effects of the SAF-containing mixtures ranged from −0.3% to +1.4% (90% confidence limits −3.2% to +4.4%) and were not statistically significant. With 25% and 50% FAME, thrust was 5.3% and 12.5% lower and EGT about 50 and 70 K higher than with Jet A-1, more than in other small gas turbines at fixed speed. Consistency checks made bench drift an unlikely cause, but the fixed test order prevented a full separation from changes during the session. The reference runs agreed within 0.7%, whereas a similarity relation predicted 6–10% less thrust on the warmer 2024 day.