Was there some preparation that was provided to the troops which we are certain consistently made use of mutton tallow?
Do re-enactor groups have rules which specify mutton tallow and if so, What is the basis for it?
Was there some guidance provided by the Union or Confederate government that specified mutton tallow?
Is there historical evidence that soldiers used only mutton tallow?
Page 266 of the 1862 US Ordnance Manual indicates the correct mixture for lubricating bullets:
is made using beeswax and "tallow". In this section, it does not specify what kind of tallow, but elsewhere in this manual in multiple places it specifically says "mutton-tallow", and differentiates this from pig lard.
Remember also, that the soldiers did not "use" lube as you may be thinking about it - the bullets came pre-packaged as "cartridges" and the bullets were already dipped in lube.
Period documents like the Ordnance manuals are primary documentation for reenactors.
You and I have been consistently on the same page with regard to bore lubes. You use Crisco because you know it works for you. I use Crisco because I don't know any better but I am about to find out.
Mostly I use Crisco because when I got in the N-SSA I was told to make my lube out of Crisco and Beeswax, so I made up a big batch of it and I have not run out yet in 2 years - I've gone through about half.
So I haven't had reason to try much else yet.
I experimented with the Civil War era lube recipe because I was experimenting with making period-correct cartridges.
Edit to add: Think modern shooters tend to use much gooey-er lubes than in period for two reasons: The softer lube makes for softer fouling, which can allow you to shoot more shots before the fouling impedes loading, and we don't have to worry about our lube soaking into paper cartridges and making a mess.
Period 1:8 tallow/beeswax, or even the older recipe of 1:3 tallow beeswax makes for a much harder, waxier lube that doesn't rub off or run and thus won't soak into the paper of a period cartridge nearly as badly.