Several months ago I think someone mentioned on this forum about using paper stock and oil drop test to view the cleanliness/particulate concentration of used oil. Attached is a photo of two samples I obtained. In the past I would just pull a dipstick and look at the oil, which is very thin layer. For the first 1K - 1.5K it is hard to tell the difference. I also looked at what a Short Volume OIl Change (SVOC) compared to a near Full Volume OIl Change (FVOC) (drain/refill a second time). I used business card stock for this and placed 2 drops of oil and let it sit for 12 hours. The top sample was taken where minimal time is allowed for draining, on a level surface. New filter. Similar to a quick lube place may offer. Sample taken after driving for 15 minutes at highway speeds. I estimate there is about 10-15% of the oil remains in this engine, which is a dual overhead cam Ecotec I-4, with VVT. The bottom sample was taken after a second drain and refill of the above oil and new filter. This equates to a 99% replacement. I drained it with the front of the car elevated and allowed 30 minutes to drain in two stages. First was a 1 minute drain and replace plug. I then waited 30 minutes and pulled the plug again. I was surprised at the amount of oil that came out (approximately 16oz) I also wiped out the oil filter housing on the engine which contained at least another ounce. This engine uses a filter cartridge. I took a sample at 15 and 250 mile mark but it looked nearly identical to the virgin oil sample, so I waited and took a sample at 500 miles. Even at this interval the particulate concentration is barely visible. If you hold the card stock up to light you can see even more particulates, and if you have fuel contamination, you will see light discoloration ring near the outer edge. Takeaway - residual oil in the engine varies on engine size, design and method of draining. Appearance of the oil on a dipstick for the first half of the OCI does not show much, unless you have diesel engine. The oil sample taken and placed on the card stock reveals much more than can be observed on the dipstick and better means of determining cleanliness of the oil. Performing a FVOC change removes nearly all the old oil in the engine. What are the negative results of leaving 10-20% of used oil in an engine, not knowing the level of contamination, wear metals, combustion by-products, raw fuel, sludge or the serviceability of the oil for another full OCI? Does this old oil affect the new fluid, contribute to wear metal concentration, depletion of TBN and additives, increase in oxidation rate? Some may say this is a waste of money and oil. Not that much. The same oil can be used for subsequent flushes, 2-3 times, or used in another vehicle such as a grocery getter or in yard equipment. Try this particulate test yourself and compare it to observable color of the oil on the dipstick, and finally with an oil analysis at the end of the OCI.