Is that...could it be...oil paint??
That is Torrit gray and white with some Gamblin solvent-free gel medium. I changed my mind on the consistency I wanted, so I thought I'd go all out and get experimental.
So I busted out my Cold Wax medium. I've no idea how long it's been since I've opened this. It's quite old and has clearly dried out a bit, but it's still usable, mostly. Seemed to go pretty well with the gel medium; I wonder how it will dry now.