Hi I have a 1991 penny as you can see where a lot of the letters are missing or have faded away. Even the date is hard to see the second "9" and the "1" on the end of 1991 would this be call a grease filled die.
The letters never actually faded away. Because of the debris filled die the letters and numbers did not form properly when the Cent was struck.
Some people choose to keep them, but over the years, you will probably find so many variants of grease-filled dies that you could fill a 5-gallon bucket with them. Chris