I dont want in the debate over oil viscosity because I know next to nothing bout it. I just use what the manuals tell me to most of the time.
Last Sunday I had couple hours to waste before the Colts game(which they lost) so I thought I'd go split some cherry I had already cut into rounds. I had left my splitter along a lane about half a mile from the nearest road. I had not given the weather much thought other then I knew it was like 12*F outside with little wind. So I throw my Carhartts on and head out. Grabbed the starting fluid just in case there was any trouble. I pull the plastic cover off my splitter,check the oil lvl, and grab the starting rope. I give it a pull and could barely pull it out. I looked the splitter over and checked that valve wasnt engaged and couldnt find nothing wrong. I gave a little squirt of fluid and gave it a yank. Same thing happened. Well what I'm guessing was 200 pulls later(figured at anytime the rope would snap) it purred to life. The cylinder would barely move at first but I took it easy letting it warm up and the finished what I had laying there which was bout 3/4 a cord.
From what I could tell it wasnt the engine oil at all but the hydraulic fluid. Now this will probly start a whole new debate, but I use iso 100 hydro oil. We change our hydro oil on our spray equipment about every 200 hours or so. The Ro-gator we just got rid of had a 50 gallon tank and our case has about 40 gallon or so. So I save the oil which I screen and use a magnet on. I'm not sure if its the best way to go or if iso100 oil is the best for my splitter , but I couldnt see wasting all the oil that looks and seems to perform like new.
I've had this homebuilt splitter for 3 years and have split I'm guessing 30 cords of wood with it and only problems so far have been switching the love joys with some heavier duty couplers and some welding on the beam due to wear. Its normally in the 40's or warmer when I'm splitting wood but sometimes you just gotta do what you gotta do when ya got the time to do it.