Just changed the oil in the K100 project in anticipation of its virgin run (I purchased not running, so fingers crossed).  The oil level sight glass was not going to be much help.  I put in three quarts and while slowly pouring in from the fourth, I held a small flashlight right up the glass, leaving a crescent of the glass exposed.  Worked beautifully, clear as day.

BTW, it helps if it's dark or dim in your work space.

The best and still easy solution to an obscured oil level glass is to buy a new one and replace it at the next oil change.  Simple job.

Al