The rear main seal could the problem , it could be throwing the oil on to the starter gear which drips off where you see it. There is no other oil source around the starter other then the sender. Check higher up, the side covers are tough to seal and I have had a tough time with valve covers leaking from the rear and running down the bell housing and dripping off the dust cover. After market chrome covers are really hard to seal up.
When sealing the covers, very few sealants will stick if oil is present. Use brake cleaner or paint thinner to clean and dry any sealing surface.
joe
