I use about 2 inches of 3/16" clear plastic aquarium tubing, cut clean and square at the oiler end, and then push the other end over the oiler spout. To use, grab the end of the tubing with thumb and fore finger and push it over the oiler to seal the tubing to the oiler base. Pump oil. You can see the oil start to flow into the oiler. It works great, but takes two hands, which is no problem.
If you use something that pushes the oiler ball down below the surface, it can get kicked over into the corner of the oiler by the spring and be permanently stuck there, and the oiler will need to be changed out to fix it. At least with Chinese oilers on my lathe. Been there, done that. Do not push the ball down below flush with the oiler surface. The plastic tubing cannot cause that problem.
The tubing slowly hardens over a couple months, making it more difficult to seal. Throw it away and install another piece. 25 feet of the tubing costs around three dollars...