1986 Honda Z50 4 or 2 Stroke?

I got a z50 from the trash and fixed it up it ran with only around 130 kicks (literately) at the start now starts flawlessly. is it a 4 stoke or a two stroke i thought it was 4 until it overheated after around 1 1/2 hours riding it then would bog down with the slightest throttle. my dads friend said it was a two stroke so i put two stoke oil in the gas and it ran for as long as i made it the day i put two stroke poil in a ran it for 4 1/2 hours and it didnt overheat once!! if it is four stroke why did it overheat without the two stoke oil? is it bad to use it with two stroke oil if it is a four stroke?