Help required, new member!

I have quite a high powered YB cosworth engine in my Dax Rush and have engine oil (very small) on the bottom of my gear box, right where the gear box meets the engine, is this likely to be the half moon or the rear main seal on the engine? Unfortunately on the Dax you have to remove the whole engine to do the rear main seal so I want to be sure. Like I said it's engine oil on the bottom of the gear box, I was just wondering if it was a half moon would it leak through the gear box sandwich plate causing it to drip from the gear box?