Engine oils can form acids as a result of the combustion process and oxidation. Even after draining the engine oil and changing the filter, residual acid contamination can remain in the crankcase in sludge form which in turn can corrupt the protection properties of your new oil. Adding Silverhook Engine Flush to your used oil before draining helps neutralise acid, dissolves varnish and remove sludge that ordinary draining would leave behind whilst providing friction proofing during treatment.