My father left me some coins some of which are wheat pennies. The olders ones have as the title suggests. It's almost like you could scrape it off with your fingernail... Almost as though your running your nail over rubber...
Most likely PVC. It's a by-product of plastic... try a 5-10 min soak in pure acetone and then a wash in distilled water... if it doesn't work completely rinse and repeat.
Dutch could this have been from being in tubes? (they were since probably the late 50's, then bagged in the mid 80's)
Since PVC was added to plastic to make it pliable, most coin tubes in the 50s-80s were hard plastic and didn't have the pvc plasticizers. I hate to say it , but much of the time, the "gunk" is just plain body oil, dirt, compressed pocket lint, etc. Remember, these circulated wheats were the coin of the relm for youngsters in those days ~ sticky fingers. Matt's solution sounds OK.
PVC IS a hard rigid plastic. It is not something you add to a plastic to make it soft. If these were just bulk wheats then Desertgem it is most likely right just old skin oils and dirt. Copper seems to have a natural tendency to attract and accumulate oils. Overtime the more volitile components of these oils evaporate off and what remains becomes thicker, and stickier.
THAT'S what it feels like... I'm using Dutchs method to 'remove' it... I know these aren't worth much, but to me it's fun. I don't have the money to be a heavy hitter in the hobby...
