I use synthetic blend DFI/Opti oil only. Mercury or Quicksilver.
There are lots of people that will tell you that you "can" use Mercury / Quicksilver Premium Plus oil. Yep. They are "technically" right. My manual even says that if the DFI/Opti oil isn't available, then Premium Plus is a suitable substitute. HOWEVER... it's not meant as a "run it forever" substitute for the DFI/Opti oil. It means that in a pinch, it will do, and you can mix it with your DFI/Opti oil without any problems. But it can burn a lot more dirty than the DFI/Opti oil, and I've known people with sneezing, wheezing sounding Optimax's that run the stuff. They end up changing their plugs more often that I do, and their engines generally don't run as smooth as mine does after they have used it for extended periods.
I've known people that run the Penzoil and other brands, and after a while, they have engine problems. Mostly rough idle and lack of performance. They bring their engine in and have it tuned up and everything goes back to normal. How much did they "save" on the cheaper Premium Plus or other brand? At the time, it looked like a lot. In the long run, they either broke even after the tune up, or they are in the red compared to the DFI/Opti oil user.
I'm a huge fan of Amsoil synthetic oils. I run it in everything I own... except for my Optimax. I've stuck with the DFI/Opti oil and (knock on wood) haven't had any issues since I bought it in 2005. At my last tune up, the mechanic called to tell me that I "really didn't need to" change my plugs, as they looked excellent. I changed them anyway, "just because" I thought it was the right thing to do. He returned my old plugs - yep. I could have saved some money and just left those in there. DFI/Opti oil is the best for your Optimax.
All the best,
Glenn