How important it is to drain all the oil when doing an oil change?

Question

Jettas have a fairing under the engine that makes it hard (impossible) to get to the drain plug without quite a bit of work under the car, so years ago I got an oil extractor so that I could pull the oil out through the dipstick opening.

Oil was a bit low when I started and I only got out about 3 liters of old oil, since I was low and because of the oil in the filter I know I should be expecting to see an amount come out that matches the engine's oil capacity, but it got me to wondering – how important it it to get out "all" the old oil? Is it important enough to optimize your oil change technique around this, or is it OK to balance other factors like convenience?

My thinking is that some old oil must be OK since there will always be a residual surface coat and probably some in oil cooler passages, the oil pump, and who knows where else.

Answer 1

Old oil isn't poison to your engine or to the new motor oil you are putting in. When you are doing an oil change, whatever old oil is left behind (along with the particulates floating around) will get diluted into the new oil. The less old oil there is the better, of course.

Most oil pans are designed to hold onto a little bit of old oil. If you look at them, the drain plug always sits a tiny little bit higher than the actual bottom of the pan. And when you are using the drain plug, you always reach a point where the oil isn't flowing out anymore, it's only dripping out. Drip. Drip. Drip. You could let it drip for hours and still not get most of the leftover out.

A good rule of thumb is to get out whatever comes out easily. Don't sweat it, as long as it's 80%-90% of the oil that's in there, whatever leftovers will be diluted 5-10 fold by the new oil. Worst case, you will need an oil change a tiny little bit sooner.