I believe it is special mineral oil like this:
Top Performance SafeEye for Pets - 1800PetSupplies.com
Personally, I would skip it. I've read that the oil layer that is meant to protect the eye has the potential to trap shampoo/conditioner causing more harm.
Max went through a period of corneal ulcers caused by a few things. I've found that for his sensitive eyes, using a gentle, tear free shampoo and exercising a little caution in scrubbing his head works. I'm most cautious with conditioner. Finally, I rinse really well using water that isn't too hot.