Thank you for the reply. On a car of this age and mileage a small amount of oil consumption is to be expected. This can be from oil leaks from ANY seal, from worn valve guide seals or from piston rings. I honestly don't think you have a "new" problem from what you described but remove the valve cover and insect the inside of the engine for deposits to be certain that the motor does not have deposits that can contribute to this. As I stated before if you had oil passage restriction then you would also have oil pressure issues which would cause oil light illumination as well as engine damage, these motors are great and hold up very well to high mileage but even a slight drop in oil pressure at any speed always causes damage to the engine, usually to #1 cylinder causing a very loud knock/tap and eventually throwing the rod through the block.
Marvel mystery oil is a good additive however I would not go dumping any cleaners/additives or synthetic oils into a motor with this much age/usage on it, you can do MORE harm by knocking deposits loose and create a major problem that ultimately I fear would require engine replacement.