Hi David - You are correct. Grease is affecting the devices. Some years are more significantly impacted (e.g. 1983 revere; EPU so much so that some Es are barely visible) and other years less so. Zincolns appear to be the most affected though. Keep an eye open for are strong example where many details are absent or for specimens where the obverse and reverse are affected. While they don't carry much of a premium, they are neat conversation pieces to show folks. Remarkably, a coin like that can cause more discussion about coins than an old coin and maybe create a new hobbyist.