The preservative you want to use is either creosote or copper napthanate. Both of these chemicals will extend the life to your fence posts substantially.

"Green" is the term used to describe wood which has not dried. Wood that contains water is less apt to absorb additional moisture. So, it is less able to absorb a sufficient amount of either solution mentioned above. Unfortunately, most folks do not consider treating fence posts, so the ones that air dry naturally while waiting to be sold are not in great demand, and for that reason the store where they were purchased would probably accept a return or an exchange for drier ones.

The lighter in weight, the dryer the post. Actually, most folks equate heavy wood with strong wood, but that is not the case when wet wood is involved. After-construction drying, and the shrinkage that follows the use of green lumber, can do much to diminish the appearance or quality of your project. Since you live in California, we assume that you are using redwood, and for that reason we would not be alarmed if you elected to proceed without the chemical treatment. What ever you do, don't use Douglas fir posts unless they are factory pressure-treated. Locally treated fir simply deteriorates much too quickly.