Myself, I have no intentions of changing out the hydraulic oil in my pt to anything other than what it has in it, synthetic might be much better and give a longer life, but you have to consider the extra expense of that change and a busted hose would be a big loss. If you intend to completely evacuate the oil that is in the machine and completely use synthetic oil you would have to figure out a way to get the oil out of the entire machine and the way I described is one way to do that, might not be the only way though. If a person wanted to add synthetic oil back to the machine by refilling the filter plus what little oil they lost between changes then over a period of time the synthetic oil would increase slightly with each oil change, but how many oil changes would it take to actually make that much difference? I would say it would take a very long time unless their machine leaked a lot and if their machine leaked a lot then you would have to consider if it was worth trying to prolong the life of a part that is on it's way out.
What would be the difference in price between the synthetic oil and the oil that came in the machine? I am just curious, I get the impression that it is quiet a bit more but I haven't checked that out. Tell me that it is a whole lot so I won't even entertain the thought of doing it.