As Watson said, it is a good practice that most people follow because some oils do not mix well together and most DIY'ers don't want a blown engine to deal with.
It is not too hard to do, but the PITA factor depends on the model of ski as some have little room to manuever arms/hands/wrenches/etc. You drain the tank, remove, clean out, change filter, reinstall everything, and then bleed the system. The GS isn't exactly one of the easier ones to do this on, but is still something you should be able to handle.
It is also good practice to change the small injection oil lines every couple years and inspect them often, as they tend to get brittle/crack and are often the reason for a lubrication failure. They run from the oil pump and down under the carbs.
And while we are on the subject of what you should do, if you have gray fuel lines, you will need to replace those as well. They corrode on the inside from the ethanol in the gasoline and clog the system/carb filters up. This could lead to a blown engine.
The one thing I find completely ODD is that the previous owner said he never had to add oil to it and it has 50-60 hours on it?!?! Did he only own it temporarily and only put a few hours on it? And how does he know if he is "skimping" or not if he doesn't even know what he bought?
The ski DOES need oil. But, you can block off the pump which would require you do you own premixing in the gasoline (you have to add oil to the gas you put in).
