This question is not the same as the one cited because that question is asking about the importance of getting leftover oil out whereas this question is asking about the amount left as well as how to ensure one gets the remainder out. The cited questions answers may be an answer to my question, however how on earth will people searching find that answer when it has a completely different question title? That's probably why I missed it. Maybe the answer should be copied here.
Garage recently put some cheaper oil(comma xtech) in engine and although it met acea spec for car, it was running noticeably worse(felt sluggish) than what I was originally running(motul). I've just done a drain and put previous motul back in and although it's improved significantly, it still doesn't feel as good as before - I can still feel the sluggishness and non lively characteristics of the comma oil. The engine capacity is 4l and we got 3.5l motul in there right up to max on the dipstick therefore 0.5l/12.5% of the comma still remains.
Is it normal for 0.5 liters of the old oil to remain? If not might the mechanic have made a mistake when draining i.e. didn't let drip for long enough? Would I have to just keep diluting with oil changes? According to my calculation one change would reduce the 12.5% comma to 1.5625% and a further one to 0.19% but it does seem to be pretty potent stuff.