Q: Can these be used for fine detail on an oil painting?
A: Yes.
Q: Is the ink opaque?
A: Yes it is.
Q: My white gel pen seems to be dry after little use.
A: On rare occasions the ink from the white gel pens may not flow as much as desired (same for all brands). This is because the white ink is thicker than other ink colors so it can accumulate air bubbles which prevent the ink to flow properly. However, there is a simple fix for it: pull the ink chamber out of the pen and blow forcefully into the open end (opposite from writing tip). You may need to do it several times and make sure to blow as strongly as you feel comfortable.