If you fill up vith 1.5 litre that is more or less the same you take out when open the drain plug so you have to take out the drain plug again and take out 1.5 litre that is a mix off old and new oil, so fill up again vith 1.5 litre and run the engine again to find the corect level fore 3rd time, and so you ceep on, meaning that if you have used 6 litre of oil you also have taken out some of the new oil when doing the drainig. This is to my knowledge the only way to do this oil change as far i know, but Flushing vil take out all oil in one time and replace it vith new oil. I realy dont understand that the garage can work on your car fore all day fore the price included oil, but thats your story. Sorry fore bad english.