I have Grumbacher Linseed Oil that I got from Michael's. Just a little bottle. Mix it with titanium white for liquid white, keep it clear for liquid clear. I'm sure Bob Ross' stuff has extra ingredients, but I like the idea of keeping it more simple anyway. I'm just an amateur, but the masters used just linseed oil for hundreds of years, right!

https://www.amazon.com/Grumbacher-Linseed-Medium-Paintings-558-8/dp/B001OV925I