I have a 1998 Johnson 150.I have used 87 Octane since it was new. Can I use a tank or two of 91 Octane and go back to using 87 Octane.I am heading to a remote Lake with a gas dock with 91 Octane. I have no problem pulling the boat out and heading to the gas station 10 miles away. Should I stick with 87 all the time ? Thanks