If it's been sitting for a very long time, I'd change the oil and run it awhile. I once bought a low miles (20K) 340 from a junkyard, and it was only a few years old, like maybe 3. I bet it took a week or more before it "cleaned up." I was concerned enough to phone the junkyard, and he assured me they'd stand behind it. I put 80K on that engine, freshened it up, and ran it a few more years in an old Landcruiser.
If that doesn't seem to work, run a compression / leakdown check
Watch for and suspect the mechanical fuel pump leaking fuel into the oil.
Might be bad oil seals
Other than major engine damage, like cracked block /pistons / rings and heads, it'll be "normal wear" like worn rings, valve guides and bad valve seals.
If the engine has been overheated, that can be a factor.