Answer: I think the oily residue is a result of insect feeding. When pests such as aphids, scales and mealybugs feed on the fluids within the tissues of the leaves they excrete a sticky substance called honeydew. If this substance remains on the leaves (and the fruit) of your apple tree is will eventually turn black (called sooty mold). It can be easily washed off so the fruit is safe to eat once you remove the substance. I'd closely inspect the leaves of the plants to see if there are insect pests. If you cannot find insect pests, the substance is probably a residue from a horticultural oil that the previous owners sprayed on the trees to suffocate overwintering insect pests. Again, once you wash this substance off the skin of the fruit, you can safely consume them. Enjoy your apples!
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