Will writes: An engine only needs 1-qt of oil to lubricate; the rest is for heat transfer.
I'm not so sure I agree with you there Will on a long term change interval basis. In most automotive engines, it would be tough to reliably scavenge the oil from the pan with less than a quart of oil. With hills and curves, you could easily cavitate the pump risking oil starvation and loss of film strength through air entrainment. Minor point though.
Of much more importance is that one quart of oil wouldn't contain a sufficient additive package to maintain the oil viscosity over the change interval. The dispersants, buffers, extreme pressure wear agents, and, antioxidants would deplete too rapidly causing the oil to become contaminated and next to useless long before today’s accepted extended change intervals are reached. The oil would also be exhibit severe dilution if the vehicle were subject to lots of cold starts. There would be too little margin of safety should overheating or leaking problems exist.
Only quantity can overcome that! Long haul trucks and industrial engines measure oil capacity in gallons for that reason. 4 to 6 quarts seems to be about the right number for most average size automotive engines.
Chumley