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#3320756 - 03/23/14 10:57 AM The History of ZDDP as an oil additive?
kd5byb Offline

Registered: 01/25/07
Posts: 724
Loc: Huntsville, AL
Good morning all,

I debated where to put this message. I put it here over the small-engine forum as the crux of my question is the history of ZDDP as an oil additive.

One of my hobbies is the collection, restoration, and operation of vintage engines. Most of these are small air-cooled engines, most often made by Briggs and Stratton. In one of the old-engine forums, a gentleman was looking for information on oil for a Briggs engine where the manual called out for "nothing heavier than SAE 20."

His question: was the SAE 20 in the mid-50's the same as SAE 20 today?

I answered back noting that viscosity measuring technology had not changed, but that oil formulation had changed. Using my spreadsheet of oil viscosities collected from various product data sheets, I noted that the viscosity of SAE 20W-20 oils (I couldn't find data on straight SAE 20) when hot were comparable to modern 5W-20 oils.

I also noted that before he run out and buy 5W-20 for this engine, he should consider that these old engines are flat-tappet engines, and that a modern PMCO oil may not have enough ZDDP in it. (I also noted that for an engine that isn't run under load, say at engine shows, it might not matter.)

I received a reply back noting that:

These engines were designed and in production before the use of ZDDP in engine oil. So they are fine to use on modern oil. Even many flat tappet auto engines are just fine, depends on the design. ZDDP is really only needed in some of the poorly designed auto engines from the 60's through 80's.

My questions are thus:

1) When did ZDDP start being used as an oil additive?
2) Anyone care to comment on if or not an old flat-tappet engine, such as a 1935 to 1955 or so Briggs and Stratton, will do okay on PMCO?

thanks much,
'97 Mustang 4.6L, '02 Lancer 2.0L, '07 Tundra 5.7L.
Please excuse typos - have an 10 year old son running around creating endless havoc... wink

#3320774 - 03/23/14 11:15 AM Re: The History of ZDDP as an oil additive? [Re: kd5byb]
eljefino Offline

Registered: 06/15/03
Posts: 31708
Loc: ME
Zinc is really a thing for high valve spring pressures which I doubt a briggs would have.

Since you're restoring these engines you're getting the sludge and goop out. A common worry is finding something that someone ran non detergent in and then shaking sludge loose when using modern oil.

#3320783 - 03/23/14 11:23 AM Re: The History of ZDDP as an oil additive? [Re: kd5byb]
old1 Offline

Registered: 01/04/11
Posts: 874
Loc: Columbus Nebraska
PersonallyI don't know when ZDDP came into use, but as far as using it in old B+S engines, I have been using whatever is cheapest #30 HDEO in my 1974 6 hp on a small rider since new. One thing you have to realize with small engines, is the valve spring pressure. Probably the cam isn't as hard as an automotive cam, but then the springs are so weak you can literally compress them with your fingers, so don't think you need to much if any zddp to offset that much pressure. ( P.S. I have been a professional small engine mechanic for 45 years. )
Then at the same time, I do want it (zddp) in my 2 classic cars, just to be safe.

Edited by old1 (03/23/14 11:24 AM)
2014 Fusion 2.5 engine 5/20 QSUD
1965 Mustang 200 c/4 10w30 QS defy
1964 Ford Ranchero 302 c/4 15/40 Delo
1929 ford model A No filters, and 15/40 oil

#3320892 - 03/23/14 01:47 PM Re: The History of ZDDP as an oil additive? [Re: kd5byb]
Darwin1138 Offline

Registered: 01/07/14
Posts: 393
Loc: Slimy Mudhole
ZDDP became commercially available after WWII. In 1951 the API SB classification established some anti-scuff properties that would call for the use of ZDDP in order to meet them.

So, an engine that was designed before 1951 never took into account the use of any additives, but this doesn't mean that using oil with ZDDP in that engine would have an adverse effect on it. Up until today, using a higher classification oil than the engine was designed for is OK, but using an earlier classification in a newer engine is not recommended.

I don't have the API specs for those classifications, but take as an example the ACEA oil sequences, the A5/B5 oil limits the sulfated ash to 1.6 % and the C1 oil limits the ash to 0.5%; but both oils need to pass the same valve train scuffing test with the same wear limit of 15 microns. But how the C1 oil, with less metallic additives than A5/B5, achieves the same performance? Because of the use of more expensive non-metallic additives.

An old engine doesn't need to use C1 oil, since it does not have a catalytic converter to protect, but the use that oil will offer the same protection to the valve train as the A5/B5 oil.

So an old 1930's engine will do OK with a modern oil.

Edited by Darwin1138 (03/23/14 01:50 PM)
0W-20 or death!

#3320910 - 03/23/14 02:07 PM Re: The History of ZDDP as an oil additive? [Re: kd5byb]
Gokhan Offline

Registered: 12/29/10
Posts: 2846
Loc: Los Angeles, California
Regarding antique engines, any modern PCMO should do fine. You may want to use a thicker grade though, such as 0W-40 or 10W-40. In addition to thicker oil film, xW-40 has more ZDDP as well. xW-30 might work just fine, too.

Here is all about ZDDP. I can't post the whole ZDDP review article because it's copyrighted:

The history and mechanisms of ZDDP
H. Spikes (Tribology Section, Department of Mechanical Engineering, Imperial College, London SW7 2AZ, UK)
Tribology Letters, Vol. 17, No. 3, Pages 469 - 489, October 2004

Excerpts from this ZDDP review article:

1. Introduction

Zinc dithiophosphates (ZDDPs) are arguably the
most successful lubricant additives ever invented. They
were introduced over 60 years ago, have been in continuous
use ever since and are still being employed in
practically all current engine oils. This longevity is all
the more striking since strenuous efforts have been
made by additive companies over the last 10 years to
replace them, but in vain. It has so far proved impossible
to identify any reasonably cost-effective compound
having comparable antiwear performance to ZDDPs
in engine oils.

As well as being remarkable in their performance,
ZDDPs have also been astonishingly successful in their
ability to inspire research. The last half century has
seen an extraordinary number of published research
papers describing investigations of how these additives
behave in their triple role as antioxidants, corrosion
inhibitors and antiwear agents.


It is not yet clear whether the limits of phosphorus
and sulphur in engine oils will be reduced further in
future, leading perhaps to the eventual disappearance
of ZDDP. Recently, attention has started to focus on
the possibility of replacing a blanket limit on the level
of phosphorus and sulphur in engine oils to a measure
that better reflects the tendency of these elements to
volatilise and thus reach the after-treatment catalyst
[15]. This may eventually lead to a limit on P- and
S-containing additive volatility or to a test which monitors
these species in the exhaust and thus permits
imaginative new formulations based on low volatility
additives [13]. Whatever the future however, there is
no doubt that the slow pace of reduction of phosphorus
levels in engine oil specifications over the last
5 years reflects the remarkable effectiveness of ZDDP
as an antiwear additive, and the great difficulty that
additive companies have had in finding a replacement
with comparable performance.


4.8. Antiwear properties of ZDDP

When considering the mechanisms by which ZDDP
prevents wear, it is important to note that ZDDP is
both an antiwear and a mild EP additive, i.e. it both
reduces wear and also inhibits the onset of scuffing.
This was recognised in the 1960s, when the influence
of metal type and alkyl group structure on wear and
EP behaviour were measured and compared [35,36].
Antiwear effectiveness was found to correlate inversely
with thermal stability of the ZDDP but this trend was
less clear-cut with respect to EP effectiveness [35]. Several
studies have suggested that the antiwear behaviour
of ZDDP results from its ability to form a phosphate
film while its EP response results from its ability to
form iron sulphide [53,130]. This is consistent with
other antiwear and EP additives; sulphur-free phosphorus
additives are often effective antiwear but generally
ineffective EP additives, while organic sulphides,
although possessing some wear-reducing capability are
generally regarded as EP additives [131,132]. Similarly,
studies have shown that in mild rubbing conditions the
surface film present is mainly a thick phosphate film
but that in severe, heavily loaded/high sliding speed
conditions a much thinner film with high sulphur content
is formed [53]. Thus we need to distinguish
between the ‘‘mild-wear’’ and ‘‘severe wear’’ action of
ZDDP, the latter being essentially an EP response.
This EP aspect will not be discussed in detail in this
review except to note that studies of thermal degradation
of ZDDP have shown that most of the sulphur
present in these molecules is converted to oil-soluble
organic sulphides and disulphides and that these are
well-known EP additives.

From the literature, three main ways that ZDDP
acts as an antiwear agent have been proposed; (i) by
forming a mechanically protective film; (ii) by removing
corrosive peroxides or peroxy-radicals; (iii) by
‘‘digesting’’ hard and thus abrasive iron oxide particles.
Each of these is discussed briefly below.

The most generally accepted view of ZDDP antiwear
action is that the reaction film acts simply as a
mechanically protective barrier [133]. This prevents
direct contact and thus adhesion between metal or
metal oxide surfaces and may also operate as a cushion,
reduces the stresses experienced by the asperity
peaks of the metal substrate. The relative importance
of these two effects has not been determined. With this
type of antiwear action, once a ZDDP film forms,
practically all that wear that subsequently occurs is
presumed to be that of the ZDDP film itself. (In fact,
ZDDP tribofilms appear to be very resistant to wear
and several studies have shown that once formed they
H. Spikes/The history and mechanisms of ZDDP 483
are only very slowly worn away even when the ZDDPcontaining
oil is replaced by a base oil [89,103]). In
this case, in mild wear conditions, the only loss of substrate
may be from iron oxide which has reacted to
form a phosphate film.

The second proposed mechanism of the antiwear
action of ZDDP is that it reacts with peroxides in the
lubricant; thereby preventing these from corrosively
wearing the metal surfaces present [69,70]. This mechanism
was convincingly demonstrated by both Habeeb
and Rounds in the 1980s and no subsequent work has
challenged it.

The third suggestion is more controversial. Martin
and colleagues have proposed that iron oxide particles,
that would cause abrasive wear, embed in the
ZDDP antiwear film and are ‘‘digested’’ to form
relatively soft iron phosphate, thus negating their
harmful pro-wear effect [62,64,129]. This model
appears to have been inferred from identification of
iron phosphate in wear particles and in the rubbing
track rather than any direct evidence of iron particle
digestion. It does seem likely that iron oxide from
the metal substrate diffuses into the ZDDP reaction
film to replace some of the zinc cations with iron ones
and form iron phosphates, and recently SIMS depth
profiling has been used to show that there is a much
lower iron oxide concentration beneath the ZDDP
tribofilm that on the surrounding metal surfaces
[96]. What is lacking as yet is direct evidence that
harmful iron wear particles are removed by a digestion

One interesting aspect of ZDDP wear performance
that has arisen very recently is that ZDDPs appear to
strongly promote micropitting wear. Micropitting
results from localised plastic deformation due to the
surface loadings resulting from rolling/sliding asperity
contact and it has been shown that ZDDP, because it
very rapidly forms a protective film, prevents or postpones
effective running-in of rough surfaces. This leads
to high asperity stresses being maintained and consequent
micropitting [134]. What is not yet clear is the
extent to which this is an undesirable feature of all antiwear
additives or of ZDDPs in particular.
1985 Toyota Corolla LE, 4A-LC engine, ~ 268,000 M
Toyota (TGMO) 0W-20 SN synthetic
Mobil 1 EP M1-103 filter, 90430-12031 drain gasket

#3320982 - 03/23/14 03:07 PM Re: The History of ZDDP as an oil additive? [Re: kd5byb]
Chris142 Offline

Registered: 06/05/03
Posts: 15828
Loc: apple valley, ca
The valve springs in a b&s can be compressed by hand. No need for any special oil.
02 Wrangler super-s 10w30
87 F250 proline 15w40
04 Tahoe super-s 5w30
Z400 Lucas 10w30
KLR250 Maxima 10w40
Can am maveric edge 5w40
57 case tractor 15w40

#3321002 - 03/23/14 03:22 PM Re: The History of ZDDP as an oil additive? [Re: eljefino]
Garak Offline

Registered: 12/05/09
Posts: 22611
Loc: Regina, Saskatchewan, Canada
Originally Posted By: eljefino
Zinc is really a thing for high valve spring pressures which I doubt a briggs would have.

Agreed. And 1950s ZDDP levels were far from impressive, and were not even what they are today.
Plain, simple Garak.

2008 Infiniti G37 coupe - Mobil Delvac 1 ESP 5w-40, Wix 57356
1984 F-150 4.9L six - Quaker State GB 10w-30, Wix 51515