Oil on plugs sounds like worn valve guides. The oil is seeping past the guides/seals around the valves while sitting. It accumulates in the lowest part of the cylinder.
5 quarts does not sound like too much. Out of the ~9 quarts (or 11 quarts with a front mount oil cooler), most can accumulate in the sump if it sits long.
How much was in the oil tank? If you add the two volumes and come up short, the rest may be in your cylinders and heat exchangers.
I do not think 5 quarts would be enough to leak past bad rings from the crankcase side.
I am no expert, though
