The second chart, below, is a mish-mosh of grays. I grayed down each color by adding its complement. It was hard to keep it in any sort of order with only 3 hues, so I just practiced adding different amounts of each primary to a puddle of "mud" (all 3 colors mixed together) to push the gray toward a certain hue.
I found an article on the web that is very helpful: http://guidetooilpainting.com/index.html. It is called Bill Martin's Guide to Oil Painting and it is both concise and comprehensive. He assumes you're a newbie and he tells you the nitty gritty. I would have wasted a lot of paint already without his tip about tin foil packets.
I like the feel of oils: they are so creamy and go on the canvas like butter. I have discovered that you use a LOT of white --- which is good, since white is less expensive than the colors. I have also discovered that it takes longer to get ready to paint, and a LOT longer to clean up than watercolors. I have pretty well got my system organized now though.