Much like Phoneman and Leftybowler70, I have done this for years as well. I have a partner who actually got me into bowling, and who is constantly complaining about his balls not working, and having to get a new one.
Needless to say, I have a basement half-full of balls he's donated to me that I then score like crazy with and never even alter much, other than clean them!
Since we still bowl league together, I will sometimes grab whatever he's using and start throwing it. This year, he's gotten smart. He's gone to the switch-grip thumb slugs. I can't throw those because the ball just falls off my hand!
He's just jealous cause I throw it better! LOL!