I'm in the category of over thinking itFord will use the most cost-effective means to meet the specification.
It's difficult to see the Mustang getting any better than a blend. Further, the default selection when getting a dealer oil change is blend.
Most owners are not as attentive with break-in as we are; a blend will assist with that. Many of us suspect the factory fill also contains additives, like XL-17, but I have no data to support the idea.
Edit,
These are the API specifications supporting WSS-M2C961-A1 (from the 2024 Owners Manual). While these standards list the tests and requirements, they do not specify the type of oil (dino, blend, synth) to be used, only its properties.
https://www.api.org/-/media/files/c...publications/17thed1509addendum7rev021218.pdf
https://www.api.org/products-and-se...categories-and-classifications/oil-categories
I wouldn't overthink it. Use Motorcraft blend, 5W-30. If you want to go over and above that by using a full synthetic, that's fine.
- Follow the maintenance schedule
- Use quality parts that meet or exceed the specification
- Document all activities
I later did some research, seeing what types of oil, brands of oil, supported Ford's specifications. From the three major labels I reviewed, the only types of oils that were listed as meeting the WSS-M2C961-A1 spec were blends or synthetics. I couldn't find a dino label meeting the Ford requirement.

What engine builder would suggest you use synthetic on a new engine?I looked this thread up because I was curious what OEM fill was in case I had to add before first oil change.
I always thought that full synthetic isnāt used in many cases for break-in, because it doesnāt allow the necessary wear on a microscopic level that break-in needs.
Turns out that is a myth and supposedly many cars come with synth from the factory.
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