"I strongly urge that the beginner work in oil for a year, or until he has mastered the rudimentary difficulties of landscape painting. Once these principles of landscape are mastered, through much experimenting ( in the more malleable medium of oil ) , the several other mediums of expression, such as water color, pastel or tempera will be found easier to handle."
Studying watercolor before oils teaches you very quickly, that if you don't plan very thorougly in advance, your painting will fail. It is very difficult to make corrections. For example, in general, you can go darker, but can't go lighter in an area. If you gray a color too much at the beginning of a painting, it is difficult to make it more intense. In oil painting, while you can make corrections fairly easily, painting directly (putting down the correct colors, right values, right form, etc. from the start) will result in a fresher painting. All of the scraping and correcting tends to weaken and "muddy" up the painting. So the disciplines learned in watercolor are valuable assets when painting in oils.
While it is the common thought that you should learn oil painting first, you tend to fall back on the notion that you can always make changes or correct mistakes later, which can result in a painting that is overworked. The critical skills in planning that you learn very quickly in watercolor painting result in learning other medium, such as tempera and gouache with more assurance.
One of my recent oil paintings
"West of Kaneville"
"The Wales Tree"
Mixed medium-watercolor and gouache