For top loaders, a full scoop shouldn't make things too sudsy compared to front loaders.
For stubborn stains, make a soft, yet still gritty paste and rub it into the area. Get a bucket and fill with enough hot water to cover however many piece of clothes you have and soak for about 30 mins. After the soak, drain the water and toss into the washer.
For general washing, I find that the whites stay whiter and colors stay brighter. Pit stains, grass, dirt, blood, tomato sauce, etc, I found that the paste, rub and soak works well getting them out. We've been using OxiClean for about a year now and we quite like it.
HTH
When to put OxiClean in? When our washing machine broke down a few months ago, we just added it at the same time as the detergent at the laundrymat.