Bought a new pair of 2 stroke 70's four years ago. Have used yamalube, marine nonethynol gas and Yamaha gear lube exclusively. Cruise at 45-4800 rpm. Change plugs annually, fuel filters every 100 hours and water pump regularly. At 400 hours a cylinder failed. Every 100 hours prop seals fail. Cavitation plate cracked for no apparent reason. I have bought a third engine for what it cost to maintain these two in four years. Had enough. Yamaha tells me how sorry they are. Time to leave Yamaha to the wealthy.