RE: .21 tuned pipe for O.S. .18 TM?
If the pipe can fit and be tuned to your engine, it will work. Tuning is by RPM. A ".21" size pipe is large enough to allow it to breathe properly.
You can use almost any fuel as long as it has at least 18% oil content, per O.S. Many so-called "R/C Car" fuels have 12?%-14% oil, and shouldn't be used in this engine.