Oh gosh!
Personally I dont use oxyclean as I have heard similar-ish stories [symbols, eyes, blush, pock marks, etc] - not about Fifi in particular but various ponies.
However, that being said I have read plenty of success stories with it as well.
I would guess the difference could perhaps be the general condition off the paint and pigment of the pony rather than the oxy treatment - i.e. how the pony has been treated for the last 30 years; different conditions could result in different rates of deterioration which could be affected differently by oxy.