Zinc and Phosphorus levels in oils-ZDDPlus

Motor oil, Gearbox oil, Differential oil (gearbox and diff. oil are shared in a transaxle) all have a certain amount of modifier packages. These modifiers add detergent, anti-foaming, anti-corrosion, resistance to extreme pressure (critical with hypoid gears like ours) and other beneficial properties to the various motor oils (or ATF) used in every car. Each company uses their own modifier packages added to generic oil bases to come up with what they believe is the best formulation at their price point.

Friction and extreme pressure (EP) additives, known as ZDDP (it’s a long word meaning zinc and phosphorus), have (had) been in use for over 60 years, replacing the previous lead-based lubricant package in oil, and doing a fine job. Until catalytic converters became ubiquitous and it was found that this ZDDP messed up the cats. So the amount of ZDDP in motor oil has been steadily decreasing over the past 20 years by govt. mandate. About 16 years ago it reached a low enough level to pose a danger to older classic cars with flat tappet engines which need high levels of ZDDP to prevent friction between the cam lobe and the tappet.

The cam lobes began wearing and the tappets began failing. At that time I had mostly modern cars, but I also had a '74 Dino 246 GTS flat tappet engine with that exact problem. After much research I decided on adding ZDDPlus to my motor oil, a commercially available additive package with all the zinc and phosphorus I would need. It’s worked out extremely well, interspersed with a few oil changes using Brad Penn Racing Oil, which has plenty of ZDDP but no detergents. Suitable for a track day or 5, but not as a long term daily driver. Back to the ZDDPlus for street cars. Works great on older American muscle flat tappet engines too. Not only engine oil saw a decrease in ZDDP. Manual transmission and differential oil also saw major decreases in ZDDP levels.

Does the XJ220 use flat tappets or a different design? I’m too lazy to start looking this up, and it’s easily fixable. Gearbox/differential oil is a much bigger problem as it is shared in the same transaxle case. The manual calls for XH-21 gear/diff. oil, but only Don Law has the original formula. Is it a GL-4 (not really great for hypoid gears), or a GL-5 (excellent for hypoid gears, but a history of problems using GL-5 anywhere there is copper, brass or bronze (like synchros), because GL-5 creates a strongly bonded sacrificial layer of protective compounds at the gear faces. GL-4 does that too. But the GL-5 is so strongly bonded to the gear teeth that as it is sacrificed, it takes a few molecules of yellow metal with it. GL-4 does not strip yellow metals.

Oil companies developed a new type of ZDDP a few years back that they claim will give gear oil GL-5 protection without the GL-5 synchro problems. But I have see no real proof of this in the standard FZG gear wear test, so for now it’s just a claim. However, many “experts” warn that even these so called “inactive” sulfur additives become active at over 300 degrees C.

Anyone else given this problem some thought, or come up with a solution? About a year ago I bought a case of Castrol Syntrax because many race teams swear by it. But recently I found Amsoil Severe Gear GL-5 which is marked as OK to use in gearboxes with synchros. So far it looks like the best candidate. I’d like to hear other opinions or experiences with transaxle oil, and if anyone knows the additives amounts in XH-21 I’d love to hear about it.


@Michael_Frank, this may be a question for which you have had some experience.

@David_Ahlers, too.

Under the E-Type forum there is a Feb '20 thread on ZDDP under the title “Hot Rod Oil …”. I posted some scientific publications on ZDDP and engineering API links. I particularly found the decades of research performed and discussed by H. Spikes of Imperial College London useful in my understanding of ZDDP. If you can’t access them, let me know if you would like copies of any of those citations I posted. There also is an article circulating on the web by Bob Olree, GM Powertrain Fuels and Lubricants Group titled “Oil Myths from GM Techlink”. I can send you a copy if you can’t find it readily.

Thanks very much Roger. I will look them up. My biggest problem right now is to find out which GL-5 EP gear oil I can use in the transaxle that won’t eat my synchros. Service manual not very helpful, as it specifies XH-21 (or something like that) which Castrol no longer makes.

I did a lot of research for the Ferrari club (NY region) on Dino 206 and 246 motor oils as zinc and phosphorus levels were decreasing, and those cars had flat tappet engines… A bottle of ZDDPlus at every oil change took care of that problem. Brad Penn Racing Oil is very good too. Has more than enough sulfur, zinc and phosphorus but no detergent package, so not great for street driving.

Thanks for the help!

Do you have any of the Castrol, to send to a materials lab, to be chemically analyzed?

Im an idiot. My car was never serviced. I should have told the restorers to save some oil when they drained the transaxle and sent it to be analyzed. Too late now. Amsoil Severe Gear (GL-5) is looking like the top contender as it specifies it will not harm synchros. The Castrol Syntrex I bought from Europe last year is also a GL-5 but it does not mention synchros, so I’m not going to use it.

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And according to the manual which I finally read, the engine is a flat tappet design, so the motor oil better have have a lot of ZDDP as well, or add a bottle of ZDDPlus to whatever oil you are using.