rear main seal leak

If you're sure, I'd say it's leaking...lol. The only thing that makes leaks is usually grease or oil on the surfaces when you're install it. I like to say it has to be surgically clean, including no oil from your skin. Otherwise, it will leak, it's just a matter of time. So I would say get whatever lip seal you're comfortable with, clean and completely degrease everything that seal will touch including the groove it seats in and the seal itself, carefully rub a little grease on the lip surface where it hits the crank, and install with a couple small dabs of ultra grey silicone. If you do that successsfully, it shouldn't leak.