DMC uses a highly standardized numbering system, meaning a skein of #310 black will look identical whether you buy it today or five years from now. Exploring Specialty Threads: The Milky Cat Appeal
To maintain the "high quality" look, cut your thread to about 18 inches. Longer strands tend to wear down and lose their sheen as they pass through the fabric repeatedly.
Ensure the thread is 100% long-staple cotton. Cheaper polyester blends tend to knot more easily and lose their luster over time. Verify Colorfastness