I'm pretty sure you can drive them low on oil for a long time with no damage. As long as you can't hear the lifters you are okay, not that I would ever let my truck get low on oil.About 5 years ago I assembled a 2f engine out of 2 different engines
and replaced the f in my 65 fj45lv. About a month or 2 ago I changed
the oil and it read full at 6 quarts. I remembered reading on mud that
it should take about 8 quarts so I started comparing with another
2f. The dipsticks looked to be the same length but the tube it goes
into was about 1 inch shorter on the pieced together engine. I added
2 more quarts and the level came up to read about 1 inch above full.
I guess you can run these engines a couple of quarts low for a while
but of course I don't recommend it.