Even quality motor oils can form acids due to combustion by-products, high temperatures and oxidation. Grit and sludge-forming acid residue can remain in the crankcase even after draining the engine oil and replacing the filter. This contaminated residue can affect the lubrication life and engine protection of the new oil. Adding STP engine flush directly to the used motor oil prior to draining helps neutralise engine acids while it works to loosen and remove sludge, varnish and gum deposits that ordinary draining leaves behind. STP engine flush contains an anti-wear additives to help protect engine surfaces during cleaning. To properly clean and protect your petrol or diesel engine: