Where's Larry Bible when you need him?
It'll take a genius the likes of which I doubt has been born to figure out how to keep oil in a Diesel engine from turning black. A trip around the block will turn a fresh fill of the best Delvac or Rotella synthetics black. There's no two ways about it with Diesels. Whatever damaged the rings of your number 5 piston wasn't something a roll of toilet paper would prevent.