I'm trying CAltex Power FLushiong oil right now in my pick-up... It worked quite good in terms of removing the black deposits IMO as usually when i dont flush, my new oil is black again after less than a week. When i tried it, my oil didnt blacken that quickly from the leftover deposits. I still change every 5000km and i probably will only try flushing the engine every 20,000km.

As for additives, nill. The engine flushes that do some damage to the seals due to their strong composition are the "5 minute" flushes according to a Nissan mechanic.