Is it possible for an oil to have too high a zinc content? If so, what is that threshold? What are the negative effects an engine may incur with an oil with too much zinc?
Can't actually answer that with a concrete answer. However, zinc is a basic metal that provides the primary "lubricity" to Oil.
The bearings in an internal combustion engine normally are of bi-metallic construction, one of the base metals being Zinc. And zinc is very malleable, from molecular size through cast or forged shapings . . So at the molecular scale, it provides that malleable "cushion" between two dissimular metals . .
I've been told by engineers and a lot of people a lot smarter than me that oil isn't much of a lubricant, but a vehicle for the compounds and metals that ARE the true lubricants, and in our street engines, it's also the vehicle for the detergents that are used to continually wash the bearing surfaces clean.
That being the case, I would assume that unless the zinc content became so dense it would separate from it's vehicle and break down from it's original compound(s), there's probably never "Too Much" . .
Kind of like the discussion or argument over what is the ideal oil pressure?
On the "Oil Pressure" thing, I believe that if you have 25-30 PSI at the
front main* through your RPM Range, you're good . . . everything's getting oiled. Others believe differently . .
*assuming your crank oils from the rear
Any Lubrication Engineers out there to discuss or refute the subject?
And . . On the subject of Alcohol breaking down the oil (Whatever one you're using). Alcohol is a aromatic solvent that doesn't dissolve readily in a hydrocarbon Fuel so while gasoline is mixed readily with oil, alky doesn't as well. Alcohol compounds, either Methanol or Ethanol tend to attach more readily to water molecules and in the process "Clump" into bubbles, which is not good in a system of oil being forced through close bearing surfaces at high speed . .
An oil ideally suited for alcohol will be one that can absorb or mix the alcohol and evaporate it through the operating cycle. Back in my "Nitro days" we watched really close for fuel in the oil because nitro doesn't mix well with oil either, and usually changed oil after every round. During my TAD Tuning days, we generally used Valvoline 50 racing oil and (Again) changed oil every round.
Currently, the Boss just put the "Grape Ape" on Alcohol and we'll see how that works. We haven't raced the car yet but it's been run several times and built engine heat in it, with cooling cycles in between. So far, the Amsoil looks fine . . We'll have more information on this by the end of April (I hope) . .