Spitting oil through tailpipes.

There is Venturi vacuum at the exhaust valve,
If the valve is awash with oil, and the valve seal is less than it should be, some exhaust seals seem less than they could be, then oil can suck down the guides.
For whatever stupid reason I didn't use exhaust seals on one race engine, it smoked after a number of miles.
I'd do as mentioned, if there's no oil on the plug, it's gotta be coming in the guide., Pull the header away, and you should see oil. jmo