ive seen alot of discussion about which oils are best and which additives are needed. my car is all stock and im getting ready to the spring cleaning on it and im not sure which way to go with the oil. do i need an oil that comes with zddp already in it? or will i be fine just adding some to the oil i use? also is synthetic better for our cars or should i stay with conventional ? if its best to add the zddp to what ever oil i use how much do i add is it just the 4 oz that most kinds seem to come in? one last thing which zddp is best ive seen several listed on ebay, are they different? thanks for any and all the help you guys have provided,jeff