I have an older high milage truck. every once and a while when i start it i get a tapping noise, like either lifter tap or rod knock. goes away once the oil gets pumped around. sometimes i *think* i hear it again when I rev the engine but that could be anything rattling (someone here suggested skirt slap). of course when you try to listen for such noises, you tend to imagine hearing things.....
Anyway, one of my cars from long ago had a similar problem and we was able to prolong it's life by adding STP engine treatment. the stuff is like an ultra thick honeylike stuff. My father said that there used to be a product like it called "engine honey" I've seen other items like this also.
my two concerns are A) is the only purpose of this is to change the oil weight so it stays in the small gaps instead of draining out and B) I was told that while this makes the tapping disapear, it can also keep oil from getting into smaller gaps and cause premature wear due to lack of lubrication.
not a big concern except I plan on using it to tow camper 150 miles to oshkosh airshow and i am concerned that running that long will cause the oil to get warm enough to not maintain pressure.