We use standard 10ml medical syringes, no needle, just the plastic body for lubing at oil balls.
If you look where the needle attaches there a short, slightly flared tube inside. In use that little tube helps seat the needle and form a liquid tight fitting. That same tube also fits a variety of oil ball sizes and makes a nice, drip free coupling. If you’ve got uncommonly large balls you can get bigger syringes from your local agricultural supply store. But don’t get the metal ones, they have a different needle seat and don’t work any better than the other suggestions here.
Not only are the syringes dirt cheap, they are, unlike oil cans, designed to be dropped without damage or loss of contents. Perhaps most exciting is the fact they don’t drip and it’s true! Not just printed on the box.