How do you remove the oil cap off a 85 gmc jimmy?

[misspanthersprincess]

we just bought the car and need to put something in the oil to try and seal up a possible oil leak of some kind, but can not figure out how you remove the cap. It's not one that you can twist off and we couldn't figure out of how to get it off without breaking something . Any help?
i think its a 2.8 , i heard that from the mechanic i think when he was looking at why it was smoking. Its rubber