Suppose I have an older car with various oil leaks and burning oil. Does oil in the exhaust stream poison the catalytic converter? Tetraethyl lead in gasoline was phased out years ago due to its poisoning catcons, but what aboutmotor oil additives and metal shavings worn away from a worn out engine?
Yes. It’s a serious concern.
Read the engineering specs for the various API oil specs: API SJ allows more of the cat-poisoning additive (ZDP or ZDDP depending on nomenclature and formulation) than API SL, which in turn allows more than API SM, the current oil spec for gas passenger cars.
Regarding engine wear metal particles combusting and entering the cat, I think that’s a non-concern.
Continuing the discussion above, ZDDP limits vary by API spec as well as oil weight spec. 15W40 and thicker SM oils get one spec for ZDDP while less thick oils get a tighter spec. I don’t recall if 10W40 gets the limits intended for the thick or the thin grades on this matter.
Something is telling me calcium levels may be of significance in cat poisoning, but I’m uncertain on that matter as well as being fairly sure the API specs don’t address them.
If this is a concern rather than a theoretical question, I would reccomend running a 5W30 meeting API SM spec. I can dig up some low-ZDP oils for you fairly quickly should you request, but Motorcraft 5W30 springs to mind as an oil with reasonable ZDP levels. Trop-Artic 5W30 is or was recently the same as Motorcraft 5W30, so that could save you about 60 cents per quart and get you low ZDP levels.
Can’t go by ZDP levels exclusively though, as there are 4 common formulations of ZDP additive and each one has different cat poisoning traits. I’ll dig up the comparison of those 4 formulas if I get the chance.
5W20 formulas tend towards the lowest ZDP levels you can find, but the ‘older car’ you’re referring to is unlikely to accept 5W20, especially if it’s not a Ford that has been effected by the automaker’s retroactive re-speccing of oils in most of its post-1991 products except for diesels and the 4.0L V6.
Sorry for the sentence structure above.
The automotive engineering community is divided on the issue of lowering ZDP levels. Toyota and Honda engineers have registered complaints with the API and related industry concerns that the API SM levels may not meet the long-term antiwear needs of the vehicles that receive them.