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.

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    Tell us more about your statement "because the engine had no oil in it", please. Had the engine been run with no oil? How did the engine come to be oil-free?
    – Tim Nevins
    Oct 17 '18 at 13:36
  • Any engine without oil feels incredibly revvy and lively right up until it ceases. An engine with low oil will feel like it's revving very freely compared to one with the correct level of oil. Unfortunately they won't last very long in that state. Oct 17 '18 at 13:37
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    The residual oil when doing a change is not usually an issue (never had much stay in any engine I have worked on, do give sufficient time for it to drain down though). The poor running is probably due to a lack of servicing, as evidenced by the lack of oil, when was it last checked? when was anything checked or is there a catastrophic leak which needs attention?
    – Solar Mike
    Oct 17 '18 at 16:21
  • Say your engine takes 4 quarts of oil. The garage adds 4 qt of bad oil. You drain 3.5 qt of the bad oil, and top it off with good oil. At this point your engine contains 0.5 qt / 4.0 qt = 12.5% bad oil, as a percentage of the total volume. If you drain 3.5 qt out a second time, and again top it off with good oil, it will then contain (0.5/4.0) * (0.5/4.0) = 1.5625% bad oil.
    – sam
    Oct 17 '18 at 17:23
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    @Sam thanks, this seems to be the scenaro but really gutted just did an £80 oil change now I got to go again. Oct 18 '18 at 17:04

The differences in feel between oils is going to be negligible by the time it reaches you in your seat. it is impossible to get all the oil out of a motor unless you smelt it down and dont use a drop of oil next time you build it. Just get it changed to whatever you like and try to stick to it. You should be more worried about diluting the metal particles in the old oil over the old oil itself

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