1995 Cadillac Seville Valve noise

Can be kind of difficult to diagnose accurately, it sounds like you have oil itself ruled out. There are several possibilities for top end type noise. Low oil pressure, specifically to the top end could be a possibility. The harmonic balancer clamping drives the oil pump, so if the balancer bolt is not tight enough it can allow for some slippage, that would be something to check. I have seen some lower end repairs that have created issues for top end oil pressure, one of which was where someone had sealed the oil pan without removing the transmission/crossover, and in the process had distorted the oil pump pickup tube, allowing it to draft air and result in top end noise. There are oil gallery plugs in the cylinder heads, but the are difficult to access. The front bank is under the water pump housing, and the rear bank is obstructed by brackets and hoses. There is no spec for pressure at these pressure ports, nor any procedure for checking, it was kind of an afterthought. It could be a faulty valve lifter, or follower(which I believe this era had bucket lifters that rode on the cam lobe itself). With it being a used car, you have little or no prior repair history. I would suggest removing the belts(main and water pump), and seeing if the noise still exists. Be certain that it is not an exhaust leak at the manifold. Checking the balancer torque, not sure on the exact spec, but with the base torque plus the angle, it is over 200 ft lbs. And then consider how far you want to get into it. Before I went to far, I would have a second opinion perhaps. I am not questioning your mechanical aptitude, but I don't know it either. Having not being able to hear the noise myself, it is a just a suggestion. If you are certain it is a valvetrain speed type noise, then go with it. If there is a chance it could be carbon in the combustion chambers, approach that first as it is possible as well, and somewhat simple to rule out.