I have some kind of liquid on my sparkplug threads only, it does not feel like an oil, nor gasoline, does someone know what could this be thanks. It is like dark brown color.

Welcome to AF.

It is hard to say without seeing it for my self but it probly is oil burnt or it could be coolant if you have a manifold leaking into the cylinder.

You didnt mention what make model or engine you have,
though its all relavent to all engines.

Probley could try changing them and thier wires/boots if performance is lacking.

Hope that helps.