Military surplus ammo is usually a safe bet because you're shooting a military rifle. But in a nutshell, the M14 likes 150ish gr 7.62x51 rounds. However, most Norinco M14s perform quite well (or better in some cases) with 150ish gr .308.
Something you'll need to recognize is that no M14 is created equal (for whatever reason) and that if a cartridge works well in someone elses rifle it may not work well in yours. For instance, some people have excellent luck with the South African 7.62x51 surplus out there (1" at 100 yards), but most of us are stuck with 2-3" groups at 100 yards. Results vary.