Rob, I think it's pretty 'common knowledge' that the people who design mechanical systems never have to repair or service them; at least, it certainly seems that way. Of course, the quick solution is to stuff your oil-checking rag down the hole but, as you point out it might be 'closing the barn door after the horses have bolted.'
Borescopes, of course, are nothing new. It's just that being able to use a smartphone--whose displays are vastly superior to to the low-res 'dedicated' devices--is the big improvement. With a smartphone display it's the camera on the scope that is the limiting visual factor (it's the only time I've wished I had a larger phone). It's a bit less convenient as I have to remove my camera's case to fit the cable, and I have to remember to keep the scope charged, but the time saved and aggravation prevented make it worthwhile. They're also cheaper than the traditional scopes; there are many selling (apparently) the same or similar scope on Amazon, all trying to beat the other sellers by lowering the price a couple cents. I was a little reticent about downloading the app--it's obviously Chinese--but have had no issues with it.