If you run an engine to its maximum temperature and let it sit overnight for 10 to 15 hours, won't a lot more oil drain into the pan vs. doing an immediate hot oil change and letting it drain for the usual 10 to 30 minutes? Some people say they wait much longer but I don't find it practical.
If I am correct, the main issue seems to be evacuating oil from the pan if the engine block is drained as best it can be. I'm not convinced that "stirring up" pan sludge by warming the engine will put it "back into suspension" if the oil and filter wasn't capable of holding it already. You're just spreading any (mobile) particles back into the engine channels. Then, you're left with a much shorter drain time (vs. overnight) to get it all back down into the pan. What's the real point?
It seems that any sludge stuck to the bottom of the pan is not going to move much, regardless of whether the oil is hot or cold ("cold" meaning ambient temperature; same temperature you pour new into the engine at). What is the real value of doing a hot oil change if it only allows a fraction of the time for oil to drain back from the engine block? The oil can only leave through one hole in the bottom of the pan, so shouldn't it be gathered close to that hole before you start? Why stir it up again if true sludge isn't movable?
When I pour new oil into an engine on a warm day, it seems to flow quite well, contrary to the notion that anything below scalding will creep like molasses. Used oil is thinner, if anything, so flow rates should be similar. I've exclusively done hot oil changes but I'm reconsidering. Hot vs. cold seems to come down to how well you can evacuate the pan, and how much MOVABLE sludge is actually there.
At worst, you'd be left with a thin film of "cold" oil on top of pan sludge that probably wasn't going to move anyway. At least that's my theory. Has anyone done serious experiments on this?
Jack
P.S. The ideal method would be to drain the engine AND pan overnight after the engine is hot, but that takes planning and isn't always practical. With any method, opening the oil cap to break suction should help it flow better.