Back when cars used leaded fuel, the lead and other products of combustion tore down the quaility of the oil when blow by occured. Also the quaility of motor construction in the 50'* and 60'* was not what it is now. In addition the oil at that time did not have the additives it has now to prevent breakdown.
In the late 70'* and early 80'* with the use of unleaded fuel, and better engine construction, oil change and tune up freq. changed. Instead of oil changes at 3000 and tune up'* at 12,000 miles, it has grown to oil changes at 7500 and tune up at 100,000.
Another thing to check is an article in Consumer Reports back in the early 90'* which tested many brands of oil. They had a fleet of 60 taxis in New York City. All 60 taxis had new rebuilt motors installed at the same time. Half of the motors had the oil changed every 3000 miles and the other half had the oil changed every 6000 miles. Guess what? There was no difference between the motors that had the oil changed every 3000 or the ones that had the oil changed every 6000, when the motors were removed after 100,000 miles of service.
Moral of the story:
change oil every 3000= more money in the pockets of the oil companies
Every car I've had in the past 10 years have all gone over 100,000 with no engine problems. Guess what-I changed the oil every 7500 to 8000 miles and most of them are still on the road well past 100K. One of them has now went past 192,000.