Hi Khorne08,
The eye could have several different problems: foreign object; a scratch; sap from a plant; infection; dehydration.
We use mineral oil for stubborn shed spots, but not for putting in our chameleons' eyes.
Our recommendations would be to try misting with warm water for several minutes - or put the cham on a plant in the shower. Aim the flow of water against the wall - this will splash water into the plant for your cham. The water should be slightly warm (test on your wrist). Mist or shower like this at least 3 times a day until he opens the eye, and then perhaps only once per day for 2 days after. This will hopefully dislodge any object.
If it is a scratch or infection, then Terramycin ointment or Gentamicin drops should be used. Terramycin is available at your vet's counter, and at some pet stores (no prescrption necessary). The antibiotic drops will require your vet.
Apply a small amount of the ointment to your pinkie finger tip, then gently smooth over the cham's entire eye turret. You do not want a glob - but you want a coating on the entire eye. We apply the ointment twice per day. You can do this after his shower or heavy misting.
If he is dehydrated, the showers/misting will do the trick - and you will know that he requires more moisture. If there was something lodged - the showers/misting should help and he will not need to rub his face on the branches.
Check your plants too... some ficus (and other plants) can excrete sap when a leaf or stem breaks off, and this can irritate cham eyes. The Terramycin will usually help in this case too.
Hope this helps,
Morgana - Reptayls, Ltd.