I've been using high mileage oil, should I stop now and switch or will that cause more issues?

 

When you stop it can cause problems with seals, you might be lucky. If it is burning oil it will kill the cat.

It is more a matter of cheaper to fix the problem in the long run rather than relying on magic formulas. If you have leaking seals replace them, if you are burning oil find out why and fix it.