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.