c# selected change event of combo box

private void cboOilVehicle_SelectedIndexChanged(object sender, EventArgs e)
{
  if (cboVehicle.SelectedIndex == 0)
  {
    whiteFusionOil();
  }
  else
  {
    silverFusionOil();    
  }
}

Edit:

The name of the control must be cboOilVehicle (Line 1) or cboVehicle (Line 3), it can't be both. You have to decide which is correct

Try this below

if(cboVehicle.SelectedItem.Text == "White Fusion")
{

whiteFusionOil();

}
else 
{
silverFusionOil();

}

If you are going to be comparing the text directly, use the Text property of the combobox:

private void cboOilVehicle_SelectedIndexChanged(object sender, EventArgs e)
{
    if (cboVehicle.Text == "White Fusion")
    {
        whiteFusionOil();
    } 
    else
    {
        silverFusionOil();    
    }
}