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There have been multiple change since the 70s that have effected engine longevity. The more complete combustion of fuel, with the use of EFI and high energy ignitions have significantly reduced the contamination of oil. The fuel its self has had a significant reduction in contaminates.
2012 Fiesta 5W20
2006 GP 5W30
If I had a 70s Escort that had been running 10W40 and had a garage full I would continue using it also.
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