Intake Manifold Gasket Leak?
 

I have a 1991 Caprice Wagon that burns oil, especially at high RPMs. My question is whether this could be caused by bad intake manifold gaskets or is it more likely cylinder oil leakage. It has around 115K on it and I don't know what the engine maintenance history is. I plan to also do a wet/dry compression test to determine what the piston/ring condition is.

manifold gaskets would leak externally, not burn internally. do the spark plugs have oil or ash deposits?

I have not yet pulled the plugs so don't have that info yet. I did put the intake manifold and rocket covers. The rocker cover gaskets were not sealing correctly. They were blue and may have been the factory installed gasket, 30 years and 115K miles ago? This resulted in external oil leakage. The intake manifold gaskets were also past their prime, allowing external oil leakage and I think also oil leakage into the intake ports. There was also a great deal of carbon buildup in the center coolant passages, which I was thinking means an EGR valve problem? I'm thinking of plugging the EGR vacuum tube and sealing the manifold port off as well. The intake replacement gaskets I have are the ones with added rubber seals front and back.

Should I be using a gasket dressing or RTV sealant on the intaake gasskets?