Typically, one of the deciding factors on whether to go with a round thumbhole or an oval thumbhole is the shape of the thumb itself. Some bowlers do have relatively round thumbs, where others like me have flat thumbs caused by years of bowling. I have tried using oval holes, but I could never seem to get an oval hole that was cylindrical top to bottom (the holes usually came out with some taper to them), so I've gone back to having round holes drilled and I shape the hole oval with bowlers tape.
If you do have a somewhat oval shaped thumb, and you want to control the release of the thumb, an oval drilling might accomplish what you are looking for. You can try it out with a slug, and if you don't like the oval drilling, you can have the slug drilled out and go back in with a new slug and round hole.
Edited on 5/9/2003 0:48 AM