First I no longer run Cox .049-.51 TDs, .074 (without running to workshop I think that is the right size) and any of the HOT engines. I ran the TDs way back in '50s and '60s using regular cox fuels when I flew FF and 1/2A proto speed.
Now in this time period I do every so often take down some of my many-year-old self designed 1/2A Cox .049 CL models in the 20-22" w/s profile and small fuselage models to fly with some of the kids around. Some 15 years or so ago, a good engine person in a model magazine now gone for years wrote that many small engines were expected to need lots of nitro. He said that 10-15% nitro was good for sport running and what was needed was more castor oil as small engines needed more lubricant than normally provided.
I now fully believe what he said is true. These Cox .049s are so easy to run well when I went to using regular 10% nitro, 18% oil RC fuel with enough castor oil to bring the oil up to 25%. Start easily, fly steady runs and no problems sometimes not having been run in over a year.
So depending on what you are after will certainly make a difference. Just thought that I would bring this item to light because it works very well for me. Good luck!