The M113 5.0L Mercedes engine valve covers and crankcase breather covers are notorious for leaks. I've read it's almost always a result of the o-ring groove not being clean enough. The breather covers don't have an o-ring and are supposed to be sealed with a specific Mercedes sealant. It's like trying to squeeze asphalt out of a tube. I've recently seen Permatex Gear Oil Gasket Maker for differential covers recommended. On my 3rd attempt, I used Red Hi Temp RTV. Not the prettiest result, but stopped the leaks.
If you have to redo this job, you might make certain the o-ring groove is spotless, but also use the Gear Oil gasket maker in a thin coat on the oil pan rail itself.
In my case, I just want the leak to stop. Pretty doesn't enter into it.
Hopefully, in your case, it doesn't leak again.