Ash is the unburnable residue that remains after oil burns. This is primarily NOT from the lubricating oil itself, but from the metallic additives in the oil. For a simple example, ZDDP additive leaves zinc behind in the accumulated ash when it is burned. (This detail by itself should be enough to illustrate the insanity of buying low ash oil then dumping in a zinc additive.) The need for low ash oil in IH tractors, from everything I have see, comes into play with the larger 6-cylinder gas engines in the 460/560 and some later tractors. These engines migrated from the truck line. The problems come from ash deposits on the valve stems.
ZDDP is a friction modifier, added to oil to reduce wear on sliding surfaces, mainly needed for flat tappets in the lobes of camshafts. The need for high ZDDP was mainly driven by some of the high-strung GM V-8s that came along in the late 1950s. They had stiffer valve springs that kept the valves from floating but also started wearing the lobes off the camshafts. None of this is a problem for older low-speed engines, or even many of the newer engines that were not designed for extreme high-performance. As you can see, a zinc additive is not an alternative to using low ash oil. In fact, it makes the ash situation worse.
Now, to the general subject of putting do-it-yourself additives into motor oil, it is generally a bad idea. The additives the oil companies use are PACKAGES of different additives. They are chosen to work together to get the desired performance. If you dump something else in, you will upset the balance. For a made-up example, adding a detergent may interfere with the viscosity index improvers and reduce the viscosity. So I generally say to buy a "good" oil and don't add any snake oil. Special situations may call for adding something, but only if you can clearly state the exact reason for doing so. For example, on old tractor that has been parked for a decade or two may have stuck piston rings. In that case, you may dump in a cleaning additive (like Seafoam) and run it that way short term.
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