The use of organic fatty acids as additives in historic mortars presents a technical challenge, particularly when oils need to be added on site. This study presents a solution to this problem by formulating hydrophobic lime putty enriched with sesame oil and evaluating air lime mortars containing the new lime putty. The results show that the mortars containing the new lime putty exhibit substantial hydrophobic effects that are comparable to those of mixes with conventional lime putty and additive oil. The soundness of the oiled lime putty was confirmed 12 months after synthesis, demonstrating its long-term viability. The proposed method offers a feasible approach for in-situ applications and contributes to the preservation of historic masonry structures.