Before I knew any better I was running 87 with echo premix in my husqvarna backpack 150 bt for about a year until I heard a slight knock. Did a little research ( back off the premix ) and realized I'm supposed to run 89. Been ever since even with my new/old husqvarna's and my echo hand helds with no knocking noise or problems. I have a friend who landscapes and uses 87 goes threw echo trimmers alot with problems. So what do you guys run grade wise? Does it really matter?