Whoa, guys...
Be very careful of "Marvel Mystery" oil in a modern engine. Some of the comments you read on the internet about products such as Seafoam and Marvel Mystery oil are pure BS.
It would be better to do a more detailed analysis of the noise before you go looking for a "mechanic in a can". Such an analysis should include using a good listening device such as the Lislie 52750 automotive stethoscope.
First, if the problem is valvetrain, you're going to hear the noise at half the speed of the engine because for each two crankshaft rotations the camshaft rotates once. If the noise is coming at the speed of the engine, then you need to consider sources other than the valvetrain.
Second, it's possible there could be a stuck lifter but, it is less likely if the engine has a low or modest amount of miles on it than it is if it's high mileage. Also, if the engine's been maintained properly (ie: regular oil and filter changes with good oil) it's less likely the problem is a stuck or "varnished" lifter.
Third, if this noise began after some high rpm, high load use, I'd would be looking more for engine damage rather than a stuck lifter.
Lastly, if you're going insist on pour-in additives, know that Marvel is basically a low viscosity oil with a very high detergent content. Other products may contain solvents. Either of these significantly decreases oil viscosity and, since you're probably already using a 5W30 you want to be very careful how you run the engine after you introduce a product that reduces viscosity. I would add the product to the engine then start it and let it idle for a while. You can drive the car but I'd run the engine at light load and low rpm and not allow the oil temp to get much over 200 deg.
After a short period with that stuff in the crankcase, I'd change the oil and filter.
if the noise is unchanged, I'd look for engine damage.
FYI, just recently, I had a friend in my club with a 93, which developed an engine noise. While the noise was coming from the lower rear of the engine, I ID"ed it as a valvetrain problem. He had others look at it and they decided it was the dual mass flywheel. He spend a ton of money and time on a new dual mass wheel and that wasn't it. They finally rebuilt the motor and on teardown, found the #8k intake lifter and cam lobe had failed. The message here is diagnose first, then act.
The stuff seems to loosen gunk and varnish and help clean the combustion area. I also add a few oz to the gas every few months or when I know that I'm going to be pushing the motor in extremes. The oil in the gas provides added top-end lube and helps keep valve guides/seals clean. When a lifter (basically a small hydraulic cylinder) sticks it can fall short of its minimum height and this can cause a mild "tic" as that valve cycles. Its the same noise that solid lifters make when they need adjustment. Hydraulic rollers are supposed to expand enough to keep that gap filled, when they are clean and working right.
