Saliva, seminal fluid or the mixture ...
In all honesty, I don't really like either. I want saliva (mine or hers), seminal fluid (pre-cum) or the mixture (especially when it drooled out of her mouth). Both are not an issue from a consumption standpont (unlike oil) and it's more or lubricating than water.
But if I have to choose between oil and water, it's water because oil shouldn't normally be consumed. Although I want a constant stream of water or a bath to keep the friction to a minimum. Tit-fucking floating breasts in water is very, very interesting. Probably the only thing better would be tit-fucking in microgravity.