car running like crap

[mmmmm_jdmness]

i would like to kno what happened i kno a guy who was running conventional brand oil in his car and then he put in synthetic and it started smoking a little so he put in nosmoke and the car started dieing little by little, harder to rev then it died he drained the oil and put syenthetic back in and it ran but sounds like **** what happened to it is it a hard fix or just costly plzzz let me kno