Running light oils, including MMO and especially ATF, can be detrimental to your engine if you are not careful. Don't run these alone, and never use more than a small percentage mixed with the correct weight of motor oil. Marvel and ATF are not intended to carry the heat and shear loads of a running engine. Damage to rod and main bearings takes only a few moments, and what would you be proving anyway? If your motor needs a flush because of accumulated carbon, there are shops that will do this for you without damage to the engine.
First on the list of things to do is to verify your diagnosis. You believe the noise is from valves, but you also say this is heard at highway speeds, suggesting you don't exactly have your head under the hood when listening to the noise. It is almost impossible to diagnose motor noise from inside the car. If you have excessive valve clearance, you should hear this noise to one degree or another at all operating speeds, including idle, so listen carefully to the engine in your driveway. Rev it up while listening to see if the noise changes or comes and goes. If you can't duplicate the noise while stationary, it is probably not the valves. If you do hear a noise, isolate it. Get a stethoscope for cars. They can be found for under $5. Be certain it really is the valves.
Knocking at highway speeds is more often a timing versus fuel issue than valves. Using the wrong gas, using too hot a spark plug range, or an excessively dirty combustion chamber can contribute to detonation and/or pre-ignition. Timing too far advanced causes pinging, and older vehicles don't control this like the new ones do. Pinging gets louder the more you push the engine, so higher speeds and acceleration loads make pining worse. Ping can occur at idle but is almost impossible to hear.
Even running a motor on unleaded that was intended to run on leaded gas can lead to knock issues that, while it involves the valves, is not a valve issue per se until your valve seats are shot. It is a fuel issue. A '72 Scout should be built to use unleaded, so that should not be a concern, but octane ratings have changed dramatically since your vehicle was built, and the chemicals used to achieve octane ratings are completely different now compared to 1972. You could try some octane booster to see if the noise goes away at speed.
Also possible if you have hydraulic valves (and I assume you do) is a sticking valve lifter, but this is usually only one or two lifters, not the whole motor, and it usually comes and goes or occurs at any speed including idle. If your hydraulic lifters are going flat at speed, which is possible and would cause a lot of valve clatter, you have an oil pump or circulation issue that you should solve fast, as significant loss of oil pressure at speed will kill your engine.
Almost all available engine oils these days are high detergent oils, so looking for a "detergent oil" to "clean" the motor is not going to get you anything you don't already have, and lighter weight oils do not clean a motor any better than heavier oils. Detergent does not add to viscosity; viscosity does not effect detergent action. Back in the early '60's it was a different story. Then you could buy oils with no detergent in them. These days you have to buy certain "racing" oils to get something with no detergent additive package already on board.
