Alan, why the coats of motor oil? I've run Highland Games (recreational level) at some of the Black Powder Rendezvous that I attend for several years now. I generally cut my own cabers for the events. I've always just painted the ends to keep them from checking too badly while they dry, but I've never treated the body of the caber itself.
I would think that the oil would interfere with the adhesion of any paint or varnish used later, although I could see where it would help "weather proof" the caber. A long time back, farmers used to soak their axe handles and such in coal oil for a period before putting them to use. Reportedly the absorbed oil helped strengthen the wood and keep it from breaking under stress by lubricating the wood fibers (don't know if there is any scientific verification of that). Would that be one of the functions of the oil?
On a related note, several years ago I was at a small Scottish/Irish festival where they had a heavy athletics demonstration. I was in the front of the crowd watching the caber toss. When the caber hit the ground, I heard it crack. When they came to retrieve it, I said "Hey guy, you broke your stick". At first they didn't believe me, till they went to pick it up. It was cracked perpendicularly, about 1/4 of the way through, roughly 3 feet from the bottom. The crack then ran longitudinally about 1 foot each way. At that point (and I actually witnessed this), they tried to duct tape it. I warned my son-in-law to move the grandkids back, figuring things might get ugly. Fortunately, the first pick proved to them that they had to go get the other caber off the van. I guess if it worked, the duct tape company could have had one heck of an ad.