Nobody is mentioning this so I will... Once an engine has used a fuel of a certain nitro % DO NOT Change it. If it was run on 20% stick with it. The materials used to manufacture rc nitro engines are designed to be ductile to facilitate compression in a ringless engine. What I mean by that is the piston relies on interference between the top of the sleeve and the piston to create compression. The ductile properties allow the piston and sleeve to mate to each other and work as a unit. Because of the ductile properties of the materials, i.e. aluminum and brass, the nitro content of the fuel used in the engine will cause the materials to harden due to being "worked", i.e. slight deformation, and "take a set". If you change your nitro percentage you run the risk of catastrophic failure due to the fact that the higher percentage fuel will cause the materials to try to "re-set". This causes cracking and eventually failure. The piston/sleeve can lose pinch, the conrod can fracture, the crank pin can be severed...
DON'T CHANGE PERCENTAGE!!!
So, yes, in essence your engine can
read the label. If you doubt me look it up in Xtreme RC Magazine. I don't remember the issue but Derek Buono did an article on that very subject quite recently.(within the last year and a half.)