I've had great success with macadamia nut oil and also apricot kernel oil. I know others have used various oils like walnut and safflower and gotten good results, too. I don't know about Udo's or flax...perhaps someone else will give you input. The only oil that I have heard does not work is coconut oil...but I have never tried it and don't know the reason.
I like using the clear or very pale colored oils...and then seeing the dark green stones. It confirms for me that these stones are not merely congealed oil!