HE detergent is properly dosed per the soil level of the load, which size of the load typically has a bearing on that (larger load logically = more soil compared to a smaller load). The amount of water involved is not so much a factor.

If you add oxyclean, just add it to the tube before you load the clothes. I too have this washer, dont use oxyclean, because I never found that it did anything better. Ride with bleach powder will do circles around oxyclean