Mercury and Quicksilver are one in the same. They are exactly the same oil, just in different colored bottles. The premium plus is a synthetic blend.
The manufacturer CANNOT void your warranty if you don't use their oil! You DO have to use the weight of oil that the manufacturer specifies, such as 10W30, etc. Of course if you have a four stroke motor and put two stroke oil in it.....you are on your own.
I personally use and highly recommend amsoil. It's by far the best oil. 100% synthetic, almost no smoke, lower engine temperature, much better protection. In my opinion, why would you want to use anything but the best? You can buy amsoil for about $20 a gallon plus reasonable s/h.
My two cents.