The other way for oil to reach the sump is a weep from the oil pump. The drive spindle has no seal, later A65 pumps have an O ring here. Wear on the spindle housing, where the spindle runs, offers a pathway. Loose and weeping backplates, poor gaskets and mating faces plus porous castings all contribute. An expensive new pump would be expected to fix these shortcomings, but it is a pricey way to go.
The oil pump cavity fills, oil vents to the sump through the timing bush clearance and the keyhole in the crankcase. But a poorly performing ball valve is by far the number one suspect, cheap but awkward to replace on a standard engine. An in line tap is a simple cure, but carries the risk we all know....
Another aspect is that on long term storage with a wet sump the crank will be bathed in oil and the bores will get a good wetting from oil spun from the flywheel. A drain plug sump plate offers a quick method of clearing the sump, but for some reason is not universally popular.
Swarfy.