Hi Steven,
I have no information about oil pastels, I've never used the medium. Not that it's not a good one, it is.

If you figure it out and make your own I would love to add an article by you on my site.

I would start with pigment and an extender like marble dust, mineral oil wouldn't dry, which is good.

Linseed oil would keep it turpentine based where you could use bees wax and maybe some caster oil to keep it from drying. A little turpentine would make it easier to blend the mix together.

Mineral oil would need a petrol solvent and might be the way to go since you could choose the viscosity of the oil.

I'll put this on my forum and ask for information.
Don