heavy weight oil will mask a small problem sometimes. Thats why they even have that stuff that looks like honey and comes out like 90 weight and you just add a quart.I saw the car when it was for sale and passed on it for certain reasons. Sounds like a rod knock on what your describing. Good luck and I hope its just a loose rocker. Try taking something like a long screwdriver and put the handle to your ear and the point to each side of the valve cover and you might be able to hear the tap. Slowly move it along the cover and you might be able to pinpoint the sound.