The oil you use will be the predominant flavor in your mayonnaise. Strongly flavored olive oil, for example, will make a pungent mayonnaise. For general use, a balance of fruity and mild oils, such as olive and vegetable, is most satisfying. Usually 3 parts mild to 1 part fruity oil is about right - although sometimes it can be half-and-half. The oil must be very fresh. One tinge of rancidity and the mayonnaise will be all but inedible, so taste the oil before you start.