Jean, you might want to get Ralph Mayer's book, an old standby that is probably on the bookshelf of most of the artists here:
The Artist's Handbook
In it he talks about which pigments dry fastest and which contain more oil. He also emphasizes the importance of thin layers of paint and talks about paintings over the past hundred years (when using thick paint first became popular) not holding up as well as paintings which had thin layers.
An eighth of an inch seems like a very thick layer of paint to me.