Who told you it is harmful? It is only harmful if it is a Diesel engine. If it is a petrol (gasoline) engine then Petrol is what it takes to run.
Yes, petrol/gasoline will severely damage a diesel engine.
If your question is about harm to the engine, and it is a gasoline (Petrol) engine, the answer is "none". If it is a diesel engine, and you put gasoline/petrol in it (like my wife did once) it will soon come to a stop. You will need to get a tow to a dealership and have the fuel tank emptied. It should not have any problems thereafter. If your question deals with harm to the environment, that is a completely different issue and beyond the scope of this forum.
Not a big deal, really. Just drain the tank and you are good to go. However, if you put gasoline(or petrol, how you call it) and start the car, you will probably need a new engine.
Yes, never put oil formulated for a diesel in a gasoline engine.
It can effect engine performance and fuel burning efficiency.
Gasoline in a Diesel vehicle will do some serious damage to the engine.
If you put petrol in a diesel vehicle do not start the engine. Petrol will do serious damage to a diesel engine. You must drain all the petrol out and flush the fuel lines.
Petrol is called Gasoline in the USA.
What happens is you must remove the petrol immediately. Petrol or gasoline will destroy a diesel engine.
Ordinary 87 gasoline, the same you put in a car. It depends if it is a four stroke or two stroke engine. If the engine has a separate filler for oil then plain petrol will do. If there is no separate filler the 25 to 1 petrol /oil mix should be OK.
No.
It will overheat the engine