If you remove and clean your rocker cover and sump then fair enough. But sometimes the flush can dislodge otherwise harmless lumps of shite. As a case in point my uncle ran a flush through his rover diesel and within a few miles suffered camshaft failure due to oil starvation. On stripping down a lump of what can only be described as varnished oil residue had blocked the oil way. I've also read on t'internet pretty much the same.