Acute aspiration occurs when enough food gets in the trachea to completely cut off the baby's breathing, which causes death in a few seconds. Aspiration pneumonia occurs when a small amount of food is sucked into the lungs, and can often be successfully treated if caught early. I personally have tried several brands and prefer Exact. I use boiled, filtered, or bottled water, avoiding straight tap water. Even in older babies, the correct consistency of formula is like thin gravy.
Thicker formula tends to cause slow crop emptying.
The correct temperature is 102 - 108 degrees. Temps above 110 can cause crop burn, and temps below 100 can cause the crop to empty too slowly. If you consistently find the crop still has some food in it at feeding time, and there don't appear to be any problems, it's probably time to feed less frequently. If the crop is emptying slowly from a mild case of sour crop or too-thick formula, applesauce is a natural emetic. If that doesn't correct it, head for the vet.