In Robert Sikorskys 1983 book, DRIVE IT FOREVER, he cites the reasoning as being the
1] the level of contaminants maxing out;
2] the depletion of the additive package;
3] changing the oil on the "upside" of this curve (expressed in miles/time);
as being the best bet for conventional oil. He also goes into discussions of how to assign "miles" to hard-driving (desert towing) or super-severe winter service as being ways to help determine oil change frequency. Is the oil clean or dirty? Did you just drive twenty miles down a dust-choked road. Then just change it. Etc.
Good book, overall.
This is consistent with what I recall from the 1960's. Lack of any maintenance was the killer of those old "cheap" cars (people kept cars an average of four/five years; car was paid off in three). Stuck chokes, worn plugs, dirty air filters, clogged PCV systems were common.
Came across plenty of examples of 2-mos/2000 oil changers and -- no surprise -- their cars ran like new. But recall this was in the heyday of full-service gasoline stations. They'd gas it, wash it, have the oil changed and everything else done for you at least as fast as Gyp Lube. Same went for tires, batteries, inspections, brakes, etc.
I really miss those stations. You took your business to them, and they took care of you. Nothing like having a real mechanic check over your car on a regular basis, familiar with you and the use your vehicle got.
As to the Imperial, my 1971 Chrysler manual states 4-mos or 4000; still a good bet for conventional to cover the max, year-round.
Although we don't discuss it here (that I recall) time is potentially more important than miles.