I would not say that Rem Oil causes 'crud ring' I would say it can contribute to the existence or the build up of the ring.
I agree. I stopped using Rem Oil in the bore years ago. It's great for the action, but I noticed it had an adverse effect on fouling with T7, Pyrodex, and GOEX.
Now here's something to ponder. As most of you guys know, I've been using Liquid Wrench as a patch lube in all of my ball guns. I have not experienced
any fouling problems with it. Nor any first shot flyers (though I do use an alcohol patch before loading). Go figure!