Engine flush is added to the old oil shortly before draining, so that sludge, varnish and deposits are loosened and carried out with the used oil rather than left behind in the engine.

It is most worth doing on engines with an unknown or patchy service history, vehicles that have run long intervals between changes, engines that do mostly short trips where moisture and fuel dilution build up, and before switching from mineral to synthetic oil.

Use it properly. Add the flush to warm oil, idle the engine for the time stated on the container and no longer, and never drive the vehicle on flush since the oil is deliberately thinned. Drain while hot so more of the suspended material comes out, and always fit a new oil filter afterwards, as the loosened debris ends up there.