My 2020 208 1.2 130hp (100kW) takes 3.5 litres of oil. After pump extracting the old engine oil, without an oil filter change, I refilled with 3.4 litres to the top mark of the dipstick. That would imply that I only left 100ml of old oil in the oil filter and engine. This doesn’t seem realistic. So instead, I reckon that the filter would have kept 300ml of oil in it (it is positioned vertically with the opening at the top).
So my calculation is that I changed 91% of the engine oil.
The purpose of this oil change is to freshen up the oil to prevent damage to the ‘wet’ cambelt.
You can use my Amazon link below to find and purchase a Pela 6000 engine oil extractor pump.
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 1.2 Puretech Peugeot/Citroen/Opel/Vauxhall Engine')){kind=link}
I use a 12v car oil extractor pump, got it off eBay, works a treat, just connect to the battery, for some reason I can't post the link.