With the engine sitting, all the oil is drained into the pan. The pan should be large eonough to hold all 4-5 quarts. Your dipstick mark for full show’s what this level should be. First step - are you over-full?
I suggest you pull the dipstick and hold it alongside the dipstick tube, cap on the tube, as it would be normally, and mark this level on the engine.
Envision that level front to back on your oil pan - depending on engine tilt, the level may or may not be above the pan/block joint at the rear. If it’s not, the pan is likely leaking either at the drain plug or the dipstick entrance. If above, could be the gasket
, but some westies have no rear seal - just a spiral groove which directs oil back into the pan when rotating - resting level should be below that point.....I had a w30 like that - would slosh out some oil in rough seas - I just kept up on absorbant pads....
Another thought - you didn’t mention if the engine’s been run - the oil filter
holds oil, and if it isn’t properly sealed, you could get seepage there till it’s empty - and will refill when you run it. On mine, the oil filer seat was bolted to the block with a gasket
- had leaks
there and had to replace the gasket.
Final place I can think of, my w30 had an oil cooler bolted to the block - gasket there could be bad as well.
If you clean the engine off well, you can also wrap the bottom with some crepe or toilet paper - it will show where the leak is coming from.