Geez, I hope my 60's Ralph joke didn't offend you. Just an oldman's humor. Anyway, if it was Valve seals or valve guides you generally don't notice it except on start up after it's set for a while (leak down). That's why I say that it's more than likely old rings and lots of mileage and the 300 motor was kind of note for it. (although the alunimum (sp) heads could be a cause). JMHO