Oil on a coticule did this to my NOS Otto Deutsch
No idea why. Afterwards my coticule exploded into a fine dust and coated the whole room. Can't explain that one either. But it's bad, really really bad.
IRL though I just finished honing an ERN I bought off Ebay, I used oil on the coti. I can't imagine what all the ruckus is about, a little dish soap cleans it right off.
I used a little diamond spray with no ill effects either. I am pretty new to coticules and ended up finishing the razor on some lapping film because I didn't have it quite right and was done playing with it. I see potential though.