I didn't think there was any such thing as high-zinc oil. Rotella diesel oil used to be higher in zinc, but now I think all oils are about the same when it comes to zinc content.
I think all I would like to know at this time is the overall length of the original shroud and the overall length of the new Brock shroud.
If the Brock shroud is 5.8 inches long, you will have about 1 inch hiding behind the dash. The original shroud may be longer because perhaps the Buick, Oldsmobile, or Pontiac varieties require a longer shroud.