I 've seen some poeple from youtube drop very oil into thier rc engine after using to maintain it.Do we really need this or the engine has enough oil which comes from fuel(cause in nitro fuel,it already mixed with oil ).If we really need this …so what can i ask for this at hobby store,,give me some good brand

What you are seeing people put is after run oil. No this is not necessary to put in your engine if you are not storing it for a prolonged period like over 2 weeks to be safe. Some people don't even use it and their engine run fine. You can put it in after every run but it is not needed. It will help keep moister away from your engine which will harm it and keep it lubricated. Any brand is good just buy the cheapest one. You can even use WD-40 but that is that good, but will still do the job. Just ask your hobby store for some after-run oil.

Any more questions please feel free to email or IM me.