Do you know how much is in the oil already, what other additives are in the oil and for that oil and additive mix what the required extra zinc may be needed. You can put too much in which starts things going backwards. Just purchase an oil with what is required. Shell Rotella T4 which is only available in 15W-40 brew. Amzoil's Z-Rod oils are formulated for flat tappet older engines that are not required to have cats. This has a few weights available. Using one of these oils you can forget about an additive and whether you put the correct amount in.
