Only additive I use is a cleaner/solvent for very dirty engines. I've seen some real cruddy insides on some cars. One test I use when looking at used cars is to stick your finger inside the cam/valve cover and rub the underside of cover. If the cover is dirty on the inside the rest of the engine is dirty too. I usually reject the car because oil changes were neglected. I helped a lot of folks out over the years with "tune ups" and when I heard a lifter clattering, or found dirty insides, I added a can of CD-2 or something like it after first changing the oil. Modern engine oil has plenty of additives to maintain a clean engine if it is changed when needed. Not when the book says or when you have a coupon for a special at Jiffy Lube.
