What causes oil on the threads of spark plugs?

Answer 1

There are a number of possible reasons behind this, but the more common reason is that you a driving a older or higher mileage vehicle and the piston rings are wearing out. As the piston rings in an engine wear out, the seal that once formed between the pistn and the cylinder wall gradually opens allowing for oil to flow into cylinders which hits the spark plugs and eventually causes "blue smoke" in older vehicles because at that point your vehicle is actually burning oil.