oil based clay to use

I use Klean Clay for my sculptures. You can order at sculpt.com or other places. I order directly from the company that makes the clay.
Reason for this- I use grey clay-ex-firm, You cannot buy the grey clay in ex-firm unless you ask for a order from the company, you have to buy 350 lbs.
I think it is worth it for the money you save in the long run.
You of course can re-use the oil-based clay over and over again.

Helpful Hints, working with styrofoam- under the clay

Saving on the cost of clay and for the sculpture to weigh less use styrofoam over the armature.
When you use Styrofoam under the clay, the helpful hint is to melt brown wax and then brush on the Styrofoam. Someone else said they put Saran wrap over the foam and them apply the melted wax.

Hope this helps.