Aaron Franklin says he uses water based slathers because they are " soluble " on long cooks. But he uses olive oil on steaks.
Meathead's PHD's say that marinades and smoke do not penetrate the meat enough to be a concern, if I recall correctly, they say only the top 1/16" will be penetrated. They say meats are not like a sponge. And smoke accumulates on the surface.
Me, I've not done the scientific tests

, I'm no scientist, I don't even have a laboratory. But I only use olive oil on steaks and I only use the squeeze butter on chicken, which the skin on yardbird is pretty tought to penetrate, anyway. As I said above, mustard is my go to.