On the suject of only using OEM parts, heres a little story.
A service engineer came in to fix a bit of kit and needed a few bits and pieces. I was asked if I had them, I didnt.
So he went off to B&Q and Maplins and bought the bits he needed to fix the machine. We got charged for the bits at the OEM costs, which I raised hell about.
I even asked a certain regulatoy body, who shall I say are firmly glued to the fence, not just sitting there.