I dont know how to kill the normal survey_crate prefab and replace it/spawn a survey_crate_oil at the same position

Pretty obvious, you need to store it's position, destroy it's object and then spawn a new object using the position you've stored earlier.

Yeah but i can't find out how to destroy the object and spawn it at the position... 

//destroy object
BaseNetworkable.Kill()
//spawn new
var testent = GameManager.server.CreateEntity("prefabname", default(Vector3), default(Quaternion)) as BaseNetworkable;
testent.Spawn();

Change testent type to your needed, set pos, rotation and everything you need

Thanks alot, I'll try it out as soon as i get home.