I've noticed some vertical play on one of my motor bells (I can pull it up about 1.5mm - DYS BE1806) and decided to rather swop it out with one of my spares. 

While I was busy I decided to give them all a proper clean and then this thought occurred...

Can / Should I oil the bearings? 

I've got a variety of oils, but the most suited would probably be the Wahl Hair Clipper Blade Oil. It is a very thin oil, and as it can withstand the friction created by clipper blades it should probably be suitable for these bearings.

Thoughts?

You'll probably get a lot of thoughts about this.  Some will say you shouldn't because the oil will flush out the grease that was put in there when the motor was assembled.  Quite right.  For me, if I have a really old motor that is starting to sound "dry" and the motor bearing is not sealed, I may put a small drop in...no more, otherwise it'll fling it every where and just attract more dirt.  As far as clipper oil, I would not use it.  I don't know if it would withstand the heat and friction of a motor that will spin in excess of 15,000 rpms.  I would use a synthetic oil that is designed for such an application.  I have  some, but dont remember where I got it, or what the brand, or viscosity is.  A google search should turn some up.  All my opinion and I am sure some will disagree.  Good luck!