Mineral verses Synthetic........
Your ski is a 96 model. The first thing I need to know is, what type and how long has the present oil been in it. If the ski has been on mineral 5 years and the plugs look good, then leave it on mineral. The bearings are already worn in. The synthetic is formualted to be a better provider of dissapating heat on the bearing surface, better than any other oil.
Mineral oil is not used to it's fullest capacity. The molecules that help it stay slippery, isn't adhering to the metals 100%, so therefore, some of the friction and heat transfter that could be taking place, isn't.
With Synthetic, it's specially formulated to be more slippery and adhere to the surface better, removing more heat that the mineral.
But if this engine has been on Mineral for the last 5 years, the plugs look good, don't change anything on it..........
The old saying......you can go from mineral to synthetic, but not from synthetic to mineral.............