I've heard this often. Anything thicker than a 23g has the possibility of releasing small chunks of black rubber butyl into your vial..You can use a smaller pin to draw it up or just avoid any chunks you cause. They will sink because they're heavier than the oil. Our rubber stoppers aren't old and they're as rubbery as they should be and if you keep stabbing them with a 21g, some rubber is bound to chunk off.