Try changing the filter, and see what happens. No matter how clean you THINK you have an engine, there is always junk left in the passages. You should never start a new or rebuilt engine without a protective screen that goes on top of the
oil filter, made especially for this. After you do your break in you pull the filter off and take out the screen. The screen will be plugged with all the junk left in the passages. Then change the filter and
oil.