Some of the more intelligent members in the field of tribology, and some of the white papers that have been posted over the years, actually showed a slight reduction in the rate of wear after an oil change, by leaving a portion of the old oil in the mix vs. as close to 100% new oil. Has to do something with not having to rebuild the tribofilm or something like that. I don't even bother draining the sump- I have a vacuum pump and marked the tube so I know where the "bottom" is. I suck it out and verify I've gotten at least 4.25 quarts of the 4.8 out of it, and then I add new oil. Every other or every 3rd oil change, I stick a new filter on... depends on the weather and on my mood that day.
My new Transit Connect is going to throw a wrench into those plans, as everything is under the dash shelf, no trans dipstick, and a belly pan underneath. Will definitely be using XG3614s so I only have to take the belly pan off once a year at the 20-ish K mark.