For me spiltting pieces that size and larger is no problem. But then again my tractor mounted splitter likely delivers more power than a 18hp. I'd recommend taking steel wedges and a sledge hammer and splitting them at least in half (maybe quarters) and then they'll be easier to roll into the splitter.
Steel wedges and a sledge hammer had more power to split than the avarage wood splitter by the way, especially in knotty pieces.
Always look for the "perfect spot" to split. This is a spot where there is no branch coming out of the piece, and perhaps a natural tiny crack (look at the grain closely). Also, big pieces tend to split easier when you take a small piece off with the splitter, from the edge of the piece, at a time.
If you put the whole thing in and try and spit it through the middle and the splitter doesn't have enough power to do it, then try splitting a small bit off the side. Every time you do that you weaken the piece.
Ripping it with a chainsaw is also a good tactic if nothing else works. Even maing a small cut in the wood, then "wedge and sledge" the crack, or put the crack against the wedge of the splitter and make the splitter do the work.
Another thing, does your rental place have a bigger splitter you can rent? I now cost is a factor, but it's worth considering, depending on how kotty and branchy the wood is.