My first engine build was an old Ford 289 in 1984. This was a class effort. There was very little oil left in the engine after draining the pan. This was a solid lifter, push rod, rocker arm valve train and of course single cam. Some may still remember these engines and simplicity. My most recent experience is with GM 2.4L Ecotec with DOHC, VVT, dual balance shafts, hydraulic lifters, oil squirters for pistons and chains, large oil galleries, and top side cartridge filters in a small 4 cylinder package. My tech said he typically adds an extra quart of oil, above what the sump requires after a rebuild. After seeing the three oil galleries and the size of the main and those in the cylinder head feed the lifter and VVT, I can believe it. Attached are photos of the cylinder head. This oil does not drain out during a typical spill and fill, and new filter. I did some oil analysis and found retention of 20% of wear metals, contaminates, particulates, and fuel dilution after an oil change. It required a second oil change to get down to acceptable levels. I estimate close to 15% oil retention for this small 4 cylinder engine. Are there any long term research on the negative affects of retention of these contaminents, oxidized oil, fuel/soot from GDI and wear metals, especially when manufacturers are pushing for longer drain intervals and reliance on Oil Life Monitors? I see an immediate decrease in viscosity, additive levels, and ability to neutralize acids in the oil within minutes of the oil change. My own investigation shows the introduction of GDI fuel dilution and soot caused a doubling of wear in the this engine generation, compared to port injected. I have started to perform Full Volume Oil Changes to remove 99% of the old oil to prevent this additional wear. Interested to know if there any research on this issue of higher oil retention.