With a quality synthetic and a Purolator, I'd go 5k without sweating.
7k? Probably fine but I'll let others who are more knowledgeable chime in.
Probably depends heavily on the year, engine, and how you drive it. (IE: if this is a 1997 supercharged 600+ HP 4.6L V8 Cobra Mustang that is drag raced, 1k is probably a stretch!)
Purolator makes a decent filter. Before I discovered the low prices and great service of site sponsor Fleet Filter who sells NAPA Gold by Wix, Purolator was my brand of choice to use with M1.
later,
b