I recently aquired a '95 Kawasaki 900 ZXI. Previous owner said to use only premium (or non-oxy) gas, and to only use synthetic oil. Is this really necessary on an older machine (runs & looks like new). I also heard that you can't mix, or switch back and forth between natural & synthetic oils. I also heard that some "synthetics" are not really that much different than natural oils anyway... there are no good industry standards to define synthetic. What is the difference between WT3 and API-TC oils?