I can’t speak t the science but I’ve used both 70 and 90 for years now and it seems to me that the 90 works faster and more aggressively. I use a generous amount of kosher salt as an abrasive as well.
Rinsing the item with warm water before adding the alcohol also helps with stubborn stains. When finished, I’ll rinse with warm water and salt. This seems to eliminate the oily film.
