I realise this is a strange looking question!
I'm in a country where many local mechanics unfortunately have a bad reputation for creating small problems while fixing other problems in order to get repeat custom. There's no real system of regulation, accreditation or formal training here. I'm pretty sure I had this problem with one garage, who I no longer use, and I'm currently working with a new mechanic who's prepared to come out to me rather than taking a car to a garage, which reduces the risks.
I've had a strange problem where about an hour after he left, having done a rudimentary fix on a transmission fluid pan leak, the car stopped starting (but did run fine 10 minutes after he left). I suspect one part of the problem might be small amounts of oil I've found in the spark plug wells, something I've not seen in this car before. This could be from an innocent small leak, or, it's the sort of thing a rogue could do quietly when I wasn't looking. He sounded less than surprised when I mentioned the problem to him just now.
Suppose someone unscrupulous did pour small amounts of oil into spark plug wells. Is this something that would typically cause car-non-starting problems immediately, or might it in stop the spark plugs operating only after a certain amount of time (e.g. for the oil to seep in), or perhaps only after the engine has run a few times (e.g. needing the motion of the engine to foul the spark plug gap)? Or maybe I'm wrong in thinking that oil in the spark plug wells is something that could stop an engine firing on its own?