In an XUD, 2 litres is enough to make sure the oil pump pick up is always under the oil level, and there is always oil pressure.
But, that's only true when the vehicle is stationary.
It's not enough oil to drive on though, as it (the oil) would overheat in no time at all, and in the slightest corner you would loose oil pressure.
when the engine is idling, the demands on it's oil are very small when compared to high load/speed, so it doesn't do any harm running on just 2 litres of oil.
What it does do is clear out a load more of the black oil that you can't drain from the galleries, and means that you haven't left thin flushing oil in there, ready to contaminate the new oil.
Works very well, and for 2 litres of oil, is worth it. I have done the same on just one litre of oil in the past, but had a slightly flickering oil pressure light. Probably not a problem, but I prefer not to see flickering warning lamps, and at £1 a litre for oil, it's not worth worrying about.
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