As any devoted X-Phile will tell you, the idea of a collection devoted exclusively to the "mythology" episodes alone is a somewhat dodgy idea, with many attendant problems. First of all, much of the show's ineffable charm was found in the balance of standalone/comedic/MOTW ("monster-of-the-week") episodes vs. the heavier mythology episodes - watching all of these tangled, knotty mytharc episodes can be a suffocating experience. Secondly, the overarching character development of both Mulder and Scully (as well as the secondary characters) was rarely restricted to the mythology episodes; someone who only knows the X-Files through these four mythology-only boxed sets will miss the introduction of Skinner and Cigarette Smoking Man's first speaking role ("Tooms"), the introduction of Alex Krycek and Mr. X ("Sleepless"), and the revelation that Scully has cancer ("Leonard Betts"), among other things.
Finally, there is some dispute as to what constitutes a "mythology" episode in the first place. For example, Volume 1 ("Abduction") of this four volume set has drawn a lot of justifiable criticism for excluding a pivotal early first season episode called "Conduit," where the depths of Mulder's obsession about his abducted sister Samantha are first explored. (The fact that "Conduit" is arguably one of the ten finest episodes in the history of the series, both eerie and poignant, only makes its omission that much more painful.) "Musings Of A Cigarette Smoking Man," concerning the secret history of Mulder's nemesis, is also a mythology episode, as is the excellent late Season Four installment "Demons." Some of the decisions made in what to include or exclude seem arbitrary.
This last point is important because it DOESN'T address the inexcusable omission of a major mythology two-parter from this set: "Christmas Carol" and "Emily." Their omission can't be explained by an arbitrary "is it mythology or isn't it?" judgment call: these two episodes are directly on-point with the larger mytharc, complete with questions about Scully's abduction, the harvesting of her ova, alien-human hybrids, green-blooded shape-changing bounty hunters, and government conspiracies. What's more, they're very good episodes, ones which I would place in the upper echelon of X-Files mythology installments.
The most surprising question, then, is: why hasn't anyone noticed they're missing? My theory: these episodes came chronologically after "Redux" and "Redux II," which conclude this set, and before "Patient X" and "The Red And The Black," which open Volume 3. The compilers must have thought nobody would notice their absence, I guess (a suspicion furthered by the fact that the "Mythology Timelines" included with these sets conveniently omit any reference to these episodes, even as they namecheck other episodes which aren't included in these packages). The reason I'm TRULY steamed, however, is because there's absolutely no reason why they couldn't have been included here. Each DVD can hold, at the very least, five full episodes (see disc 6 of the Complete Season Two boxed set), and the final disc of this set only holds three. This wasn't an issue of limited space and painful decisions needing to be made, it was simple laziness.
There have been a number of X-Files mythology episodes that were either badly written or irrelevant to the larger conspiracy. (For an example of the former, see the atrocious "Per Manum" or the Season Nine two-parter "Nothing Important Happened Today." For an example of the latter, see "Fallen Angel" and "Tempus Fugit"/"Max": all three of these are excellent, and in fact the latter - included on this set - may be my favorite two-parter in series history, but none of them have anything to do with the greater conspiracy.) "Christmas Carol" and "Emily" are neither of these, and their unexplained disappearance in these sets is therefore inexcusable. It comes close to invalidating the entire raison d'etre of this package in the first place.
This may seem like a bit of a rant, but the mistake/intentional oversight in this case is so inexcusable, and the loss so great, that it seriously damages the value of the "Black Oil" set. Unless you buy/rent/have already seen these missing Season Five episodes, you're missing a huge part of the story. Additionally, the Black Oil set is light on useful extras: there are only three commentaries (and R.W. Goodwin is soporific as always on "Talitha Cumi"), and the documentary is skimpy and inessential.
Ultimately, these mythology boxed sets are most useful for fans who want to follow the continuing storyline through the last few dodgy seasons without buying the complete seasons. In that sense Volumes 3 and 4 are the ones to get, while Volumes 1 and 2 are less so since you're much better off purchasing the new reduced-price editions of the first five or six seasons. (Also, there's far less confusion in Seasons 7-9 over what does or does not constitute a "mythology" episode - you won't be missing anything crucial with the later boxed sets like you are with the first two.)
I'm giving "Black Oil" two starts as opposed to only one because there's no denying the objective quality of most of the installments found here. "Nisei"/"731" ("Scully, let me tell you, you haven't seen America until you've seen it from a train"), "Piper Maru"/"Apocrypha," "Memento Mori," "Tempus Fugit"/"Max" - these are series highlights. But for reasons explained above this set was already a questionable investment; the omission of a major (and high-quality) mythology two-parter merely ices it.