I agree that ovals/rounds are a personal preference. I too, gave (ovals) a try and
I had no success with them. Although they were more "comfortable" than the round holes, I could not get clean, consistent releases with them. As I got older, my thumb
began to change. The skin on my thumb(and hands) became thinner, which resulted
in other issues. This caused the sides of my thumb to tear and blister whenever I bowled. Numerous span/pitch changes followed in an attempt to minimize the discomfort. The ovals dramatically relieved the abrasion points but at the expense of
encountering inconsistent release problems. No amount of realignment solved the problem for me. After numerous tries, I found a happy medium. I went back to round holes, shortened my span, reduced my thumb pitch, and added more right (palm) thumb pitch. I also use tape (as JustRico pointed out) to modify the shape
of my thumb.
All of the above have made a noticeable improvement, both in terms of
comfort and performance. I keep a very tight thumbhole in order to eliminate
the dreadful "grab" and to allow gravity to release my (thumb) where the ball is heaviest--at the bottom of the downswing, at the release point. These combinations allow me to do that better than anything else I have tried.
