I hear what you are saying. I did change the plugs. Did a compression check this past week and all cylinders were between 180-190 psi. I havent changed the oil in a long time because the engine only has about 3,000 miles on it. As far as going above 4500 rpm it won't go that high in gear right now! And I only put about 3 gallons of gas in the tank. The tank looked really good inside I was really surprised for being 33 years old.
I will drain the oil and cut the filter apart as you said and see if there are any signs.