thumb callus -- need oval slug in ball?

[GoKidd]

I mentioned to my local pro that I'm experiencing a tighter fit on all my current balls of late.
In fact, everytime I show up to bowl, I have to spend a considerable time gradually inserting my thumb until it squishes down small enough to get in there! Unnerving.
Last Monday league I developed a little blister/skin flap on the side of my thumb nearer my body.
My pro looked at my thumb and noted that I have developed enough callus that he thinks I should use an oval-shaped slug (rather than round) in everything from here out.
Is this typical for a new bowler?
Does anyone really have a round thumb profile?

[The Mayor]

Yes, there are definitely people with very round thumbs, but most people have some sort of oval to their thumb. The best option is to have your pro shop drill an oval thumb for you. Not an oval thumb sleeve. When you pro shop actually drills the hole oval instead of round, they can actually determine just how much of an oval would be best as well as which angle.

[GoKidd]

I may have used the terminology wrong.
What I've been using is a round, pre-formed "slug" and then my pro adjusts it to fit me.
The same pro was saying I should move to an oval pre-formed "slug".
Is there a "slug" that is not pre-drilled?
Sorry, but this is all new to me.

[DanielMareina]

Yes! Most thumb slugs are not pre formed. Most are undrilled, and then drilled to be specific to each bowler. Most bowlers (95%) have enough of an oval in their thumb to need oval. People also have different angles that their thumb need to be ovaled. I definately recommend getting a custom fitted thumb slug!

[GoKidd]

I definately recommend getting a custom fitted thumb slug!

Thanks, Daniel, that cleared the fog for me. Yes. Next time I get a new ball or get one of my current balls serviced, I will have my pro shop operator go oval rather than round when they fit the slug to my thumb.
I think the callus has changed my thumb shape over the past year and I didn't mention that to my pro until just recently.