I don't think it was because of the oil. If anything you will have slightly higher oil pressure which isn't a problem.

What can happen, especially during hotter weather or even if your car is running too lean, is that your cat heats up too much and the internals of the cat melt.

if you can, unbolt the cat and give it a shake. See if it rattles.

You'll be able to smell it if your cat is dead.

Jase

PS Just thought....shouldn't an emissions light come up on the dash if the cat is rooted??