I'm thinking of installing a MINI MANIA header on my '03 R53 with 60,000 miles. Should I put in new o2 sensors while I'm at it or should they be alright? Can they be cleaned at all?

 

02 sensors are too expensive just to replace just cause. Don't try to clean them you'll more than likely damage them.

Also do some homework before you buy a header.

 

Agree. Wait until they fail. They are not that hard to get to if they need replacing later.

Make sure to not contaminate them when they are out. Don't let them come into contact with anything.

I got a Milltek header. So far so good.
Cheap headers = Check Engine Light problems.