Early in my bowling career I had the same problem you are experiencing - a ripped callus where the thumb meets the palm. At the time I was trying to put "spin" on the
ball thinking that the "spin" would get me more strikes. A few seasons later I learned how to stroke the
ball instead of trying to manufacture hook by spinning it off of my hand (and thumb). Occasionally I would get a blister but knew enough to take precautions in preventing it from tearing in the middle of a bowling
game.
It was when I found a driller who introduced me to the oval thumb and fingers that my blister problem completely disappeared.
At the time I was using plastic. The driller was Jim Lewis who is now a retired PBA member.
The reactive bowling
balls are much more powerful than what you were using 20 years ago. When properly thrown they do most of the work in getting strikes. As far as your release is concerned, you get the most by doing the least. On a personal basis, in my stance I position my middle and ring fingers at 5:30 and at release those fingers are at 5 o'clock and I try and stroke through the
ball towards 1 o'clock. As a result, wear and tear on my thumb and fingers is held to a minimum. I do get a bit of callus buildup but it is very slow and before it gets too thick I file it down with a callus sander that I picked up in the cosmetic section of the local drug store.
Other factors leading to a callus/blister where the thumb meets the palm are:
A bad edge or bevel.
A misfit
ball - too long a span puts too much pressure on the base of the thumb - a too short span will cause you to squeeze the
ball causing you to hang on to the
ball by the edge of the thumbhole. When the
ball is hanging by your side, in a properly fitted
ball, the pressure of the
ball should be felt in the thumbhole against the part of your thumb in front of the thumb nail.
You may need to check the size of the thumbhole and the pitch. Too large a thumbhole will cause you to squeeze the
ball creating a blister, as will too much reverse pitch cause you to squeeze the
ball giving you a blister.