Excellent post yard-dart I suspect that we have had similar experiences. I was running 15% Morgan in my 4S's but it was a little lean on oil I thought. So I added a little castor to each gallon even though it was not recommended in my engine manuals. I just wanted to get the oil content up. I was running Enya # 3's with no problems in my Magnum 52 and OS F's in the others. One day I made a fuel run to my LHS and they had YS 20/20 on the shelf. At last a fuel with an oil content more to my liking and recommended for my YS to boot. Suddenly I started having really bad vibration problems with my little Magnum. After a lot of headscratching I switched from the Enya to an OS F. That helped a lot. But I continue to have occasional problems with pre detonating and have kicked props off twice this summer once sending me to the ER for stitches so I had to richen up all my engines considerably from where they ran on the 15% (I guess that stands to reason, more nitro and more oil means less methanol) to stop it. I need a cooler plug than the F. I am currently trying the YS fs plug and it seems to be cooler. There is a thread here somewhere on plug heat ranges, but the cooler plugs are not carried by my LHS.