let me do a short feedback after using the shell flushing oil. after i drain out the old oil, i pour in all 4 litre flushing oil and let it run for 5 mins, the clear flushing oil become as dark as my previously oil. guess it does actually clean the engine. engine does sound little smoother on idling. not bad for RM27.50, i'll used on my other cars as well now.
you can seek your nearest shell service center.
what engine/car/truck u use?
it shouldn't be a problem if you use full 4 lit.
ut if your engine is super dirty... may use two bottles...