The wife just called to tell me that there's an oil-like substance where I park in the garage. Looks like passenger-side. Anything I should be looking for?
Check the oil psi sensor that is usually located above the oil filter. Most common source of S550 leaks across the entire platform.
Check oil filter to ensure it's on tight.
Depending on year, check valve covers. Earlier 5.0 S550's lacked bolts in every boss of the valve covers. There was a TSB posted on here which addresses that and additional bolts are needed for the fix.
I don’t think it would be oil filter, sensor, or cooler if it’s on the passenger side. I just had a cracked oil cooler replaced under warranty and it dripped straight down by the driver’s side wheel