With the 0w-40, you obviously still had more than enough reserve pressure. For reference, my dad when he serviced a 1964 Galaxie, used only 30 grades. Granted the HTHS was probably closer to 3.5 than it is in modern PCMO 30 grades.
As for leakers/burners, it depends how much it leaks and/or burns, of course. The taxis went hundreds of thousands of miles on conventional at double the normal OCI. It would be hard to see what benefits I would have achieved from synthetic there. Longevity was obviously never a concern, and one can only extend OCIs so far, and I only wanted them to go so far. There's a value in getting under the vehicle for an oil change, since one is likely to check other things at the same time.
Under the worst conditions, my old LTD would be burning a quart per 500 miles. Back then, with synthetic still about $10 a quart, I wouldn't have been doing it.