Had the leaky oil drain issue this week when I swapped sumps. I tried a few different solutions mentioned (except oem) but ended up making my own gasket which is proving itself ok. I found the leak is from the bolt holes so in order to seal them up I made a gasket from radiator style rubber-fabric-rubber. This seals the bolt holes as clamping force of the bolts squashes the rubber, the fabric stops the rubber from just squashing and splitting. Bit amateur but working the best by far at the moment.