That area of the seal is supposed to have
oil on it.
Every
oil leak on almost eny engine usually shows up on the front of the block plate. It's a nearly horizontal surface on the engine that is tilted towards the rear. Mot people assume it's the rear main seal, when it usually is not.
First, check the most obvious places first - valve cover,
oil pan, etc. It could even be from the fuel pump boss or crank seal.
One weak spot on a SBF is the front cover to oil pan seal. Make sure those front 4 bolts are sealed with silicone or thread sealant. On some covers those bolt holes extend to the interior.
Clean the engine really well. Make sure everything is throroughly degreased. Let it sit for a day or two to make sure it really is dry. Sprinkle the suspect areas with baby powder or corn starch. Run the engine for a few minutes, and look for changes in the powder. You'll need a strong light and a mirror.
Or, a lot of shops have a leak detecter. It's a machine that produces a petroleum based smoke. They inject it into the engine via the valve cover or dip stick. Where the smoke leaks out is where the oil leaks out.