Zinc has been removed from oil in the last few years because the EPA says it messes uo the cat cinvertors. Some deisel oils still have it, but even Rotella has cut most of it out. Motorcraft deisel oil probabky still has it since it says not to use it on gasoline engines with convertors on them. I purposely bought some to use in my Metro, just to spite the tree huggers, lol. I have always used 15/40 or 20/50 in all tyoes oils, normal, mixed, or full synthetic. Synthetic oil will clean the sludge out of your engine so if your engine is nasty inside, you may want to change the filter early at least. STP oil treatment has lots of zinc in it so adding a bottle should bring the low or no zinc oil back to old levels. Not so needed fir roller cams but the flat tappet cams do need it.