STP oil treatment

[junkyardjeff]

I have two gallons that are over 20 years old that has zinc that I want to use in my flat tappet motors,I was reading the container and it said it was not recommended for multi grade diesel oil and since I use that in one of my vehicles I was wondering why there could be a problem. The only thing I can think of is when that STP was made diesel oil had plenty of zinc and adding it would be over kill,now it does not have that much zinc so would it do any damage if I use it with todays 15/40 diesel oil.

[Beck]

I believe you are correct that the new oils zinc content has been lowered, but cannot be sure.

Too much zinc is just as bad as not enough. There is a fine line, happy medium, with the zinc content. The company I work for is a major zinc additive producer. The stuff we make is pure, not cut like the STP. It takes 1 oz of our product to bring a 5 qt oil change up to the old content. Back in the day we made the main STP component. Unfortunately we sell the zinc additive by the rail car. No quart cans allowed.

If using any product that old be sure to shake well. The zinc could settle out.