Firstly, we can see about sludge and its formation. Sludge is a semi solid substance which forms inside the engine and deposits in the engine itself. It is formed due to various reasons. Some of the reasons for the formation of sludge is due to poor crankcase ventilation, low engine operating temperature and presence of water inside the crankcase. If sludge is formed, there are chances for engine oil getting clogged up and blocking the oil passages. If this happens, oil flow to the components will be restricted and wear and tear will become higher.
Is flushing the oil before every oil change is necessary? Let's see about that. The point here is, its good to flush your old motor oil before every oil change. But its not necessary all the time. But it is recommended for some conditions. When you didn't changed your engine oil for a long time, surely sludge will be formed and deposited inside your engine. At this time, it'll be better to flush the old engine oil before replacing it with the new one. Another one is, when water entered into the crankcase. When engine oil mix with water, the quality of the oil will be decreased and sludge will be formed and deposited in the components. At this situation also, you can flush your old engine oil before replacing it with the new one.
This flush oil prevents old oil to mix with new engine oil. When you use engine flush oil before replacing with new one, the old engine oil will drain out completely without any deposits. Old oil thickens and have more resistance to flow. When you drain without using flush, small layer of old oil will be deposited in the crankcase and in the sump. At this condition, if you add new engine oil, it mixes with the old one. Flush oil prevents old oil sludge mixing with the new engine oil and drains it out completely.
While changing the engine oil, oil filter also must be replaced. Though oil filter captures dust and soot particles, some smaller dust particles escapes and gets recirculated. By using this flush oil, you can easily get rid of this dust and sludge present in the oil.
The main function of the engine oil is to reduce the friction and allow the components to move freely. When oil transforms into sludge, it gets deposited in between the moving components. Due to this, the lubricating property will be reduced. If this happens, more power will be wasted in moving the components inside the engine which results in consumption of more air fuel mixture. If this sludge is removed and when your engine is flushed completely, you can good throttle response and optimized fuel economy.