Oil leak
 

Okay, I have this very slight oil leak @ the oil fill cap. I replaced the oil fill cap & it went away & then yesterday it began leaking again. What else can I do to prevent this from happening? (1980 450 SL)

Edit:
The oil leak is in the dead center about 12 - 18" behind the drain plug. I can tell it's the oil fill cap because I can see the oil on the outside of the head cover & it's fresh.

Thanks for any ideas on what else to do.

It is not unusual that the valve cover starts to leak a little oil. You can either tighten the bolts for the cover (not what I would do) or replace the valve cover seal. The seal is about $4 and you need 4 $0.85 crush vashers. It takes about 30 minutes to do and an easy DIY.

Thanks but it's definitely not the valve cover, it's coming from the oil fill cap, very noticeable. Is there an additional o-ring or something that I need? The new oil fill cap worked for about 3 weeks & now it's leaking again.

Your PCV hose may be blocked and allowing crankcase pressure to build up and force oil out of the cap. When the engine is running, remove the cap and see if there is pressure in the engine. If so you have a PCV blockage somewhere.