Back in the day, I drove a Chevrolet Corvair with horizontally opposed six - like a Continental, Franklin or Lycoming. Lifter leakdown overnight was not uncommon. If a lifter was still noisy by the time you got to 35 mph, you were a quart low on oil.
Collapsed lifters (lack of oil) will clatter, and give reduced power at high rpm, but not too serious if they are set correctly with about .030" gap when collapsed. The beauty of them when receiving oil is that they fill up, and follow the camshaft lobes perfectly, rather than solid lifters being hit by the cam lobes, the amount and noise of impact depending on the gap.
GB