Do I believe my mechanic?

[mrnickleye]

Any experienced tech can differentiate (?) the sounds in an engine.
Rod knock, valve lifter, crank bearing.

When your engine runs low on oil, then you are harming ALL the parts. Rod knock will likely show up.

Make sure you use 4 qts 20w-50 oil, and.....
1 bottle of Lucas oil treatment,
OR
2 bottles of STP oil treatment, at each oil change.

Trust me (just look at my credentials), this is the only way you will "prolong" the engine life, and prevent further damage.

If the engine is pretty quiet with this oil and treatment, keep it full, and drive it until you can make needed oil leak repairs, or new engine.