Oils are a component of most salad dressings, but in 1918 cooks were urged to reduce their use of fats to support the troops in World War I. And, even if they could get olive oil, it was expensive. An article that year in Good Housekeeping recommended that cooks use the “new” salad oils. Here’s some excerpts:
New Salad Oils
This is the time of year, above all others, when the palate craves the coolness and pungency of salads. Since the salad dressing is often the making of the salad, it is the chief consideration. The main ingredient of most salad dressings is the fat. We have been asked to be sparing in our use of all fats, but fortunately for us the new vegetable oils have come to our rescue.
Olive oil is becoming scarce in this country, and is, in consequence, high in price, but there are plenty of good substitutes in the cottonseed, peanut and corn oils which have been placed on the markets.
While to the lover of olive oil none of these makes quite so good a dressing as the olive oil itself, it is not difficult to prepare satisfactory dressings, and the untrained palate often finds them even better. The more refined and desirable these vegetable oils are for salad oils, the more tasteless they are. They are, therefore, excellent conveyors of condiment. If the flavor of olive oil has become a necessary and fixed habit, a dressing can be made by using one-third olive oil to two-thirds of any substitute oil. For this purpose purchase a heavy, highly-flavored oil oil.
Good Housekeeping (August, 1918)