While oil is essential for traditional mayonnaise, you can use unsweetened applesauce, avocado, Greek yogurt, or silken tofu as partial substitutes to reduce the amount of oil needed. These will alter the flavor and texture, but can help in a pinch.
Traditional mayonnaise relies on oil to create its emulsified, creamy texture. When you run out of oil, completely replacing it is difficult, but you can use other ingredients to supplement and reduce the amount of oil required. Here's how some substitutes work:
Remember that these substitutions will change the flavor and texture of your mayonnaise. Start with small amounts and adjust to your taste. You may also need to adjust the other ingredients, such as lemon juice or vinegar, to balance the flavors.
When using substitutes, blend the ingredients thoroughly and gradually add the oil to ensure proper emulsification. Adding the oil too quickly can cause the mayonnaise to separate.