The M10 while it will shoot just about any type of .38 Special bullet, was originally designed and built around the 158 Round Nosed Lead bullet. Not the best for self defense (slow velocity and lack of expansion) but is one of the most accurate, non leading, pleasant rounds to shoot.
For self defense consider one of the 158 grain Buffalo Bore, or Speer 135 grain Gold Dot offerings. They are good for their intended purposes. For plinking and target work, the 158 RNL are tops! By the way, as long as old ammo is stored in a cool dry place, it will work just fine regardless of age.
Chief38