Could be anything leaking back there, oil pan gasket, rear main seal, transmission front bearing retainer, etc... My opinion is that it's not that big of a deal unless the friction surfaces were actually oily. At 5000 rpms, and high heat not much is going to stay on the friction surfaces unless the leak is really really severe. I had a bad distributor seal oil leak about 1qt+ a month, the inside of the bell was caked in thick brown sludge, but same situation, friction surfaces were dry. The oil wasn't even causing any strange issues with clutch operation I was in there because the pilot bushing had grenaded. It was much worse than what you're showing here. It actually looks normal to me for an area that doesn't get washed as its sealed up.