[Desert1957]

Leaking valve guides are above where the valve seats on the head . They have no effect on engine compression at all or intake vacum readings.
What happens is the oil around the valve spring area seeps down the valve stem and is sucked into the combustion chamber (cylinder) on every intake stroke. This causes plug fouling over a short period of time depending on how bad the guides or stem seals are. This forms the small crusty coating on the plug that causes rough idle and misfiring under load.

You can prolong the plug life by installing umbrella style seals over the valve stems. this shields the valve guide and deflects the oil. it works fairly well untill that seal wears out. Only other way to fix this problem is to have the heads removed and rebuilt.

Desert