Do go check out Tad Spurgeon’s excellent site. Since the last time I had been there, he’s really fleshed out a method for making “putty” with various heat bodied oils and different chalks. These putties form a thick, dull grey medium that is mixed with regular paint to adjust it’s transparency and handling characteristics. Although the putty is grey all by itself, it has no color when mixed with regular pigmented oil paint.
Painters such as Velazquez and Rembrandt routinely added such materials to their paint. Tad seems well on his way to recreating some of their methods.
Aside from this, Tad has lots of great info on oil painting, including a well-written and useful introduction for beginners. Go there now.