Back a while back (which is really more like about 35 years ago!), Delo and Rotella were the two go-to brands of oil for owners/drivers of diesel-powered trucks. In some markets, Exxon XD-3 was similar. Those brands were usually available where "diesel drivers" bought fuel or got service, even for diesel farm tractors. 1400ppm zddp was "normal", although some now claim that the zddp in diesel motor oil is not the same zddp as gas-engine motor oil needs to have.
As over-the-road diesel engines started to have tighter emissions controls (for newer model year engines, at the time), the "C_-_" designations got a bit higher in letters and numbers. The zddp level was posted in the oil forums as dropping to 1300ppm zddp. Some people screamed "NO ZDDP in those newer oils", although the zddp level was still very high in them. As the gas motor oil went from "SL" into "SM" (with 800ppm zddp as the upper spec limit), more hand-wringing happened, by observation. Later formulations of diesel motor oil got closer to 1200ppm, with greater levels of detergents in them. Still above the "SL" upper spec level of 1000ppm zddp.
In the 1990s, after the "SM" spec oils were all that was normally-available, I sold a customer a new Chevy Crate Motor, a ZZ430 big block with an EFI set-up on it. I pulled the instruction sheet out of the box of goodies stashed under the intake manifold to look at oil recommendations for the care and longevity of that motor, as it had an agressive camshaft in it. It said to use "SM" motor oil in it. Now, this was an engine which had been "fire tested" (initial run-in on a dyno stand for 30 minutes, then went through the two minute final test, and then crated for shipment).
Prior to this, VW had been having issues with their automotive diesel engines, which used one camshaft lobe to run the high-pressure diesel fuel pump. If the recommended-spec oil was not used, within about 10K miles, the camshaft would be ruined. Euro OEMs use "OEM Approvals" rather than "API" ratings for their motor oil, fwiw. Many USA owners only knew about "viscosity" rather than these "minor" things, by observation. Learning was an expensive lesson for them.
As normal, "Better Living Through Chemistry" happened and Amsoil came out with their Z-Rod oil as niche brands as Brad Penn and Joe Gibbs' Driven oil came out with higher doses of zddp in them. As Mobil 1 added (or I became aware of) their Turbo Diesel and Truck syn motor oils, too. Delo upgraded ot "IsoSyn" technology along about this time, too. Sometime through this period of time, a friend in another car club sent me the file/SAE paper on GM's testing of SM-rated motor oil on a flat tappet engine (at that time, the only flat tappet engines they were producing were Chevy crate motors, as I recall). Their "measure" was the weight of the valve lifters. More wear meant less weight. Their data indicated that with the SM oils, "weight loss" was the same as it was for prior ratings of non-syn motor oil, which validated the 800ppm (SM) oil as being fine with a flat tappet engine.
Now, as this was done on a production engine of theirs, it ALSO meant the Parkerizing treatment of the cam lobes was OEM-spec, too. Something which was not addressed, per se, in any other comments on cam lobe wear! My observation was somewhat proven when a major cam company started to offer "Additional Parkerizing" as an option on their camshaft orders! NO admission that their prior Parkerizing might not have been to OEM levels, but offering MORE Parkerizing to apparently get to that level. Interesting side issue, to me! This same camshaft company, when the issue of lobe wear arose, recommended "Either Rotella T or Synthetic motor oil" for their camshafts. Later, of course, every cam brand came out with "oil treatments" to use with their cams, on a regular basis, to ensure longevity.
The cam lobe issues first arose from the rebuilder industry (using aftermarket stock replacement cams) and the race engine builder communities. ALL using aftermarket parts, by observation. Which, to me, raised the red flags about the Parkerizing treatments on those parts not being to the OEM level of things.
Now, back in the 1950s when hot rodders were getting their factory cams "reground" for more lift and duration, no real mention of cam lobe longevity, from what I could determine from listening to older hot rodders talk about things. Apparently some of these re-grinders were better than others, though. This was also a time when motor oil was inexpensive and could be changed more often, if needed. There were NO specs about zddp levels, than any commoner knew about, back then. Probably just that some oil brands were better than others. Sometime, during these times, Valvoline seemed to align itself with the "high performance" realm of things with their active sponsorship of major NHRA (and probably others) drag racing events. Look at the spectator stands and you'll probably see large Valvoline banners attached to them somewhere in the picture.
Looking at the older oil analysis reports I've found online, from the later 1960s or so, the zddp levels were all over the board, with FEW to NONE getting anywhere close to 800ppm zddp. But consider too, that OEM recommended oil change intervals were in the 3000-4000mile range, too. A few brands allegedly used the longer-chain molecule Pennsylvania crude oil for their allegedly better products. But as chemistry evolved, those smaller companies were bought out/merged into other bigger companies/brands and that orientation disappeared. Or they couldn't afford to keep up with the "super oils" of the later 1960s, when 10W-50 viscosity ranges appeared.
In modern times, most oil brands mention "superior base-oils" in their formulations, but keep which amounts of which group of base-oils as their trade secret. Obviously, some might be better than others, with the additive package tailored to the final product. An additive pack which includes "zddp" and "moly".
Here are some things to check for in seeking a high or higher-zddp level oil. NO GM Dexos approval. DO look for multi-viscosity oils with the upper viscosity number of "40" or "50" or a "straight-weight" 30 . . . ALL out of the normal OEM viscosity ranges for current production gasoline vehicles. DO look for the word "Euro" as related to the oil's viscosity, too. Castrol still makes some syn motor oils with the "SL" rating, too, which usually carry some of the older Euro OEM approvals, in the 5W-30 viscosity.
A word of caution, though, it originally was designed to be a diesel-spec viscosity range with higher zddp level, but the 15W-40 viscosity has NOW been used for normal gasoline, lower zddp, motor oils. Not sure why, but it has happened. So look for the "C_-_" diesel motor oil API ratings to make sure you get the prior higher-zddp oils. Additionally, the "Rotella Gas Truck" motor oils are NOT high zddp oils as the current Rotella T-_ oils are. On the plus side, the Rotella T-_ oils (from straight-weight, blended syn T-5, and full syn T-6) are still the higher zddp level oils, with a diesel engine rating.
Other than diesel oils, "Racing Oil" is another indicator of higher zddp levels, usually to about 1400ppm. Which includes Valvoline VR-series oils, Amsoil Z-Rod, some Mobil 1 products, many Brad Penn, and several Joe Gibbs' Driven oils. For a normal street motor oil, 1400ppm zddp seems to be the higher limit without sacrificing the needed detergent levels of the motor oil for 4000+ mile oil change intervals.
Sorry for the length. Thanks for your time,
CBODY67
