Oil grit

If that is really metal, the engine is disintegrating and is about to expire. More commonly, I suspect if you run that grit between your fingers, you will find it dissolves into a dark brown oil. That is sludge and is not that uncommon. Running at higher temperatures, especially longer highway-speed trips, will reduce the formation of that stuff. The higher temperatures keep blow-by and gas vaporized so the PCV system can draw it out to be burned in the engine. The coolest part of the engine is near the "oil" cap, so lots of blow-by condenses there. Missing scheduled oil changes and using oil that's too old can cause this too. By "too old", I mean oil that has been sitting in your garage for many years. The containers have letters stamped on them with an "SD', "SF" or some such rating. The "S" stands for "spark ignition", meaning gas engines. There will also be a "C" rating, such as "CG" or "CH". The "C" stands for compression ignition, meaning diesel engines.

It is the second letter you are interested in. Any time a significant improvement is made to the additives in the oil, it warrants the next higher second letter. You will always find oil with the latest rating on store shelves. That will meet the requirements for any engine up to that year, including all older engines. If you have a stash of oil from before 1999, it is possible it does not have the additive package to meet the needs of your engine. Those include anti-foaming agents, corrosion fighters, viscosity index improvers, detergents, dispersants, and seal conditioners. Detergents clean up the pollutants that cause sludge, and dispersants carry that gunk to the filter. We are always coming up with new additives for gas, and some of that finds its way into the oil. Oil's additives have to keep up with anything bad gas can throw into the oil.