Assuming this means they show up with a fairly thick leading edge bevel and you take a good bit off of them?
Stove, you had another post that I don't see now (Woodsplitter advise). My response fits better with this thread anyway. Prehaps you decented to go Sledge & Wedge. My vote here.
I also do not own a wood splitter but am not harvesting wood to heat. Only cut for enjoyment, exercise, & cleaning up dead/down trees, then sell after drying. Typically 12 to 15 face cord (4 to 5 cords).
There is a technique that really helps ease that 1st half round split work. Use your saw to make an end "slit" about depth of saw bar, 2" to 2 1/2". Even 1 1/2" usually fine. Place "slit" kerf cut across whole diameter 'end grain' of chunk to split, then insert Steel Wedge to center of round & bang with 6# or 8# Sledge. Usually pops into 2 halves within 3-4 swift blows. Then, axe work much easier on remaining broken halves. Can repeat quartering saw slits if desireable work reduction.
I use this technique in the woods to reduce 24" - 30" Oak to sizes I can lift & load. Then later use Maul or Axe to make cord wood for drying. I'll often cut 6-8 blocks, turn on ends, put "slit" into those, then continue blocking cuts. Later come back with Sledge & Wedge where can bust up all the blocked round in 20-30 minutes. The technique yields 2 efficiencies; 1) saves back from heavy lifting, 2) fewer pieces to handle in woods, only reduce blocks to a say 40# transport weight. Yes, still considerable work effort but zero cost or maintenance of a wood splitter. I'm age 62 & don't mind the exercise, but never a speed contest.
Tip: if wood has an exposed end grain Crack, place saw "slits" to extend those cracks across block diameter. As chunks already exhibit stress that want to release there (cracks).
My limited knowledge... there is an inherent problem using a fixed flow Auxiliary Hydraulic source. Follows many dislikes of tractor hydraulic mounted Splitters. The splitter cylinder has only one working speed; same extension & retraction cylinder speed (fixed flow rate). Often proves to be slow working speed, lower productivity. A longer cylinder retraction wait to reload splits/blocks. Self-powered splitters address this by incorporating Two-stage pumps, say 8/16 gpm. Yields 2x speed cylinder movement until a splitting resistance. So nearly twice the cycle rate. Something like 60-70% productivity improvement.
Also, I view borrowing an Auxiliary Power Unit has other issues. Seems inefficient on another scale.
My estimate, you'd want a 4" or 5" x 24" cylinder. Depends on max pressure of hydraulic source. My impression 4" is most common for a high capability splitter. Again, assumes the borrowed hydraulic source has volume capacity to supply a 4" cylinder (302 cu. in, 1.3 gallons per stroke)