Yous ski is a 951 which has RAVE valves, so you should be using an API-TC rated full synthetic oil. If you can find out what the previous owner used and it meets specs. continue to use it. ie, SeaDoo XPS, Quicksilver, Amsoil Interceptor. The rating is more important than the brand.
I'm not going to go into a lot of detail because honestly this subject has been beat to death. If you change brands you will need to change the oil filter and really you should change the filter annually.
Full synthetic should be used in RAVE valve engines, 787 and larger, synthetic blend or mineral oil can be used in now RAVE engines, 720 and smaller.
Lou