Hi,
I find that based on UOA that actual mileage and heavy loads, racing and lugging an engine wears it more and loads up the oil with wear metals, etc, driving down the Total Base Number of the oil and the Total Acid Number up.
Then I did a UOA of my sunday driver that I haven't used in 10 years, that oil still scored TBN of 5.6! with wear metals and water content that was so low it became clear that oil sitting in an engine for years, barring a coolant leak via the head gasket will still be OK.
With a late model Mustang and a newer, cleaner, tighter engine with much lower blow-by, you can do the Ford recommended intervals and longer once out of warranty, test the oil and see!
I now use 2 Filter Mags on the oil filter with 2" x 3/16" Sq neodynium bar magnets between the Filter Mags that visibly keeps the oil cleaner for up to 9-10 months and 8,000 miles before it even gets a little dark, never mind solid black!
If you can do any of that too, you got nothing to worry about!