Grease compatibility is an important consideration for end-users and grease suppliers. In the process of replacing lubricating greases, the issue is whether two greases can safely be combined or whether mixing them will decrease performance and even damage equipment. Almost 60% of global grease consumption is based on lithium soap thickener chemistry, so finding alternatives and determining their compatibility is imperative. The experimental compatibility limits of different greases in the current work were examined by preparing binary blends of a reference lithium complex grease (LiX) with three possible substitute thickener types: aluminum complex (AlX), polyurea (PU), and calcium sulfonate complex (CaSX) greases. Three blending ratios (10:90, 25:75, 50:50) for each couple were used to model partial or near-complete grease substitution. In total, nine mixed samples and four unmixed samples were tested in accordance with the ASTM D6185 standard for grease compatibility. First-order test criteria of importance were dropping point, shear stability, and high-temperature storage stability, and secondary tests included windfall storage stability and antiwear performance. The results showed that the lithium complex–polyurea combination achieved the highest compatibility score overall, while the lithium complex–calcium sulfonate complex blends were the least compatible by ASTM criteria.