I found the reference of the test I mentioned at
Here is the summary:
There was once some discussion on this list regarding whether or nor sucking the oil from the sump with a Topsider Oil Changer gets ALL the oil out.
Well...I did a test. I filled a new MB sump (thanks to Bill Alexander) with water and removed the drain plug and allowed the water to drain out. I then measured the amount of water left in the sump.
There was 500ml left - about 2 cups = one half quart.
Next, I filled the sump again and used a Topsider to suck the water out. I had cut the end of the suction tube at a 45 degree angle so it would not seal against the bottom of the sump. I kept the sump level and allowed the Topsider suction tube to suck out all it could while holding the tube vertical to the sump bottom as it would be in actual use.
There was less than 200 ml of water left in the sump - less than 1 cup..
Conclusion: The suction method removes slightly more oil from the sump than the drain method.
This test was done with a sump part number 616 014 01 22 found on almost all 240D and 300D and gas 6-cyl MBs from 1974 to 1985. Just looking at the drain plug location in the sump shows that some oil will always be left in the bottom after an oil drain, so for those of you who are anal retentive about removing EVERY LAST DROP of oil at oil change time...just get over it...;-)
I guess you could tilt your car over to the left somewhat to get another 200 ml of oil to drain out... or remove the sump and drain it completely...;-)