FLUSHING should be fine but then draining the oil completely after flushing is very important. these TASS people are in immense hurry to screw the drain bolt back and so the whole sludgy oil is not drained out. i am too planning to flush the engine as i am going to ask them to remove the sump as there is some leakage from the sump gasket. i have asked them to change the gasket. so if sump is removed 100% drain out will be there.