Many people who see my art ask me how I get the depth of the backgrounds with oils. I learned the technique that I use by painting with different artist. One in particular was a well known tattoo artist turned fine artist.
Here's how I do it:
I use mineral spirits to really 'water down' the oil paints. I use mostly Winston Newton fast drying oils (24-48 hour dry time) or Gamblin oils. Quality and pigment is important when choosing the brand of paint that works for you and your art. When I am putting on the paint I put them on in very light coats, thinning with the mineral spirits, and then let them dry before the next coat. The dry time depends on how thin the paint coat is that you just added but usually 2-5 days. With most of my paintings in which I use this technique I usually paint 5-10 layers depending on the piece. I blend in the layers so you can't see where the strokes end/begin but you can see the variation in saturation.
Note - Don't make the paint so thin that it drips. Also, if you try to put the next layer on before the previous is dry you will take off the previous later. Those areas are hard to fix so test if it's dry with your finger on the side border of the piece before adding the next layer.
Hope this helps!!