The newest spec for engine oil is SL and one of the
differences is that it removed all of the zinc from
the formulation. It is correct that zinc has been one
of the primary reasons for cat failure. As the engine
ages and it burns oil the zinc would degrade the cats
and hence its efficiency. Zinc's primary purpose is
an extreme pressure lubricant, therefore it prevents
wear on cam lobes, lifters and main bearings. Less
zinc more wear on parts. Diesel oil still contains
zinc but it is available in 15W-40 which may not be
suitable for your car. A better solution is GM EOS
(Engine Oil Supplement) which still contains a high
amount of zinc. You can use GM EOS in your newer car
also. It is not harmful to your cat if zinc is in the
engine only if it is being burnt. GM EOS seems like a
better solution if you are worried about the lack of
extreme pressure lubricant. The following was taken
form the Castrol Oil web site as to why you might not
want to use diesel oil in your gas engine.
Can you use gasoline engine oil in a diesel car and
vice-versa?
Because gasoline engines and diesel engines run on
different fuel, they have different characteristics
and therefore the performance requirements of engine
oils must also vary accordingly.
Dispersion cleaning properties (washing out residual
particles such as sludge to keep the engine interior
clean) and anti-wear protection for the camshaft are
especially important in gasoline engine oil.
Furthermore, gasoline engines are equipped with
catalysers to purify exhaust fumes and an oil
formulation that prevents adverse effects to these is
essential. Diesel engines, on the other hand, produce
more soot and so an ability to control the soot
build-up is crucial. Also important are antacid
properties to neutralize the acidic particles that are
produced by the combustion of sulfur present in diesel
fuel. As particular emissions are more controlled
vehicle manufacturers are designing engines with
emissions equipment that requires high performance
oil.
Consequently, it is not advisable to use gasoline
engine oil in a diesel engine, or diesel engine oil in
a gasoline engine. The SL, SJ, etc. performance
ratings of gasoline engine oil all begin with the
letter 'S' (Spark ignition), and the CF, CD, and so on
of diesel engine oils with 'C' (Compression ignition),
so it is quite easy to tell them apart.
[Gasoline/Diesel dual use] engine oils that carry the
rating [SJ/CF] have been specifically formulated for
both types of engines and can be used in either,
according to conditions of use.
One other point, zinc is just one of many chemicals
that can be used to provide extreme pressure
lubrication. The others are: organic phosphates, acid
phosphates, organic sulfur and chlorine compounds,
sulfurized fats, sulfides and disulfides, none of
which are restricted for use in SL rated engine oils.
As zinc was removed that others were added to make up
the difference, the problem has been that the
manufacturer's have changed how engines are designed
and built and need less extreme pressure lubricating
properties than before. I think the answer to the
question has now become use a good quality SL engine
oil with GM EOS supplement if you want additional
extreme pressure lubrication properties. Additional
information can be found at
http://www.bobistheoilguy.com/ The site is loaded
with up to date information on engine oil from some
knowledgeable lubrication engineers.
=====
Bill Strobel
Independent Towing
Fayetteville NC
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