I would be careful to use 15 percent oil in any motor, even the chrome ones. If you are having good luck with 15 percent you must be keeping the motor tuned right and staying a little on the rich side. My personal preference is 20 percent oil. The extra oil does not affect performance and it gives me a higher level of confidence against an accidental lean run. Running 15 percent oil in any engine is certainly a testament to its durability.