When I was still working in the trade the thought process was this....if you change the oil and filter at the correct intervals then you shouldn't need to flush the engine as most particles will have been collected by the filter and will come out with the old oil. The problem with flushing is that it can disturb larger particles which can then build up and block oilways thereby causing you a much bigger problem in the end. Just my ten penneth...It's always up to the individual and if you ask ten people you are liable to get ten different opinions...