Chief, first you need to find the leak. If oil is on the starter, it most likely didn't come from the rear main seal. Your first thought was the filter housing.Get a mirror and a flashlight and check all around the oil filter housing and block/head interface. Have the engine running for this so that oil is being pumped.
I once had a sizeable leak (stready trickle at idle) at the filter cap gasket after a shop used a cheap (e.g. Purolator or Fram) filter whose included rubber o-ring was undersized. Check down around the base of the filter housing as well.

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Prost!