FWIW; I would like to share my thoughts on this subject since the use of oil and lube has been cussed and discussed since I got my first Muzzleloader over 55 years ago.
My interest in Muzzleloaders has waxed and waned over the last five decades, but when I got "real interested" in shooting muzzleloaders, sometime in the late 1960's...early 1970's, there was simply not enough information available.
Most folks, myself included, relied on some local expert to tell us what, and what not, to do.
Of course, at the time, there was no Internet and darn few books dedicated solely to the use of Black Powder firearms, so the owners of such books as The Muzzleloading Caplock Rifle by Ned Roberts, and The Kentucky Rifle by John G.W. Dillin, became the subject matter experts overnight....great books, both of 'em, but the experts created from simply reading these books has left something to be desired.
However, once again according to Ned Roberts, other than these two books, nothing had been published in the United States since 1848....
Long before many of us older folks even got started in the game, technology had already provided the world with a modern steel and the "traditional" metal used in barrel making was a thing of the past.
As Captchee so aptly pointed out, modern day Steel and the Iron used in the past, require different treatments and herein lies the heart of this age-old discussion.
The two great books mentioned above dealt mostly with guns and barrels that were available from the days when iron barrels were still used, seasoning was a very viable term, and "oil" meant rendered Bear Fat or Sperm Oil....almost exclusively.
Going back 50 years, I think we can easily see how the modern or new and the old became intermingled.
What with most written references of the day being directed one way, ie toward older barrels, while we were in fact dealing with the new, more modern barrels.
Also, going back fifty years, it was that thought at the time that if you read it in black & white...it was a fact!
Soooo, in view of all the above ramblings, I believe, very strongly, that today's modern made guns do require oil for a preservative and rust inhibitor.
I do not believe we should avoid "petroleum based" products, as long as it is removed before shooting the firearm.
Patch lube is a hoss of a different color as the properties of Black Powder itself has not changed that much.
Just my thoughts.
Uncle Russ...