Some good ones are 50% synthetic polymer base, and more. Some bad ones are as little as 20%. Furthermore, it came to my attention a couple of years ago that Castrol semi-synthetic brews are actually full-synthetics with a cheaper additive package in them. Make of that what you will. What I make of it is that labels like "Semi-Synthetic" and "Fully-Synthetic" now have more to do with performance than what's actually in the base oil product.

With regard to the whole business of seal degradation, that was an issue in the old days, to be sure, but shouldn't present a problem in anything built after the late '80s or so. Modern additives are fully and properly compatible with the seal material.

When you mention "real fully synth", are you perchance referring to pure synthetics? These are a different animal altogether, not for automotive use, and you won't find them on the shelves!

 

Dear Melchie, the real fully synthetics I referred are still automotive products but the so-called fully synths came from a famous court case where I believe it was Mobil who claimed their product was a fully synth when in fact it only had (from memory) 20% 'real' synthetic oil.

This lead to synthetic oils being classed by the amount of real synthetic material in them.

If you Google synthetic oil+court case+Mobil you should find more info.


On a more general front I use semi-synth in my cars but do oil changes every 5000mls.

I use "fully-synth" on my bike as it runs harder and at higher revs.

"Due to the court case in the states between Mobil and Castrol, you may not
always be getting what you think you are so be careful, hydrocracked oils are
not synthetics in the true sense of the word as they are molecularly converted
petroleum oils, synthetics are not, they are built by chemists in laboratories
"brick by brick" and are far superior.
Unfortunately, apart from in Germany, a manufacturer can label the inferior
"hydrocracked" oils as synthetics and therefore the only true way of working
out the quality is price although even this is not certain as there are some very
expensive "hydrocracked" oils out there which are sold on their brand name,
Castrol is a good example as they were the Company that Mobil took to court
over the labelling issues.
Here is some more reading for those interested:
“HYDROCRACKED” (HC) or MOLECULARLY CONVERTED (MC)
BASESTOCKS
There are many petroleum oils available on the market that are so pure and
refined, they can now be passed off as synthetics.
They are not made from true synthetic basestocks (at least not in the way that
synthetics have traditionally been defined), but they have so little in common
with traditional petroleum basestocks, it is really somewhat silly to classify
them as petroleum oils.
Petroleum oil basestocks can be put through a super-extreme refining
process called “hydrocracking”. In some cases, as in the case of one
particular name-brand "synthetic" oil, these highly refined petroleum
basestocks can actually be termed and sold as "synthetic".
It is completely legal for lubricants manufacturers to label these oils as
"synthetic".
These are extremely high performance petroleum basestocks, but they are
not truly synthetic the way that most people understand the term and will not
necessarily perform to the same level as a premium synthetic oil like PAO
(poly alfa olefins) or Esters.
Hydrocracking involves changing the actual structure of many of the oil
basestock molecules by breaking and fragmenting different molecular
structures into far more stable ones. This results in a basestock which has far
better thermal and oxidative stability as well as a better ability to maintain
proper viscosity through a wide temperature range - when compared to a
typical petroleum basestock.
Although contaminants are still present, and these are still petroleum
basestocks, contamination is minimal and performance characteristics are
high. This process also can turn a wider range of crude oil stock into wellperforming
petroleum lubricant basestocks.
TYPES OF SYNTHETIC BASESTOCKS
Synthetic basestocks are not all the same. There are few different chemical
types that may be used as synthetic basestock fluids. There are only three
that are seen commonly in automotive applications:
Polyalphaolefins (PAO's)
These are the most common synthetic basestocks used in the US and in
Europe. In fact, many synthetics on the market use PAO basestocks
exclusively. PAO's are also called synthesized hydrocarbons and contain
absolutely no wax, metals, sulfur or phosphorous. Viscosity indexes for nearly
all PAO's are around 150, and they have extremely low pour points (normally
below –40 degrees F). Although PAO's are also very thermally stable, there
are a couple of drawbacks to using PAO basestocks. One drawback to using
PAO's is that they are not as oxidatively stable as other synthetics. But, when
properly additized, oxidative stability can be achieved.
Diesters
These synthetic basestocks offer many of the same benefits of PAO's but are
more varied in structure. Therefore, their performance characteristics vary
more than PAO's do. Nevertheless, if chosen carefully, diesters generally
provide better pour points than PAO's (about -60 to -80 degrees F) and are a
little more oxidatively stable when properly additized.
Diesters also have very good inherent solvency characteristics which means
that not only do they burn cleanly, they also clean out deposits left behind by
other lubricants - even without the aid of detergency additives.
They do have one extra benefit though, they are surface-active
(electrostatically attracted to metal surfaces), PAO’s are not “polar”, they are
“inert”.
Polyolesters
Similar to diesters, but slightly more complex. Greater range of pour points
and viscosity indexes than diesters, but some polyolester basestocks will
outperform diesters with pour points as low as -90 degrees F and viscosity
indexes as high as 160 (without VI additive improvers). They are also “polar”.
Other synthetic basestocks exist but are not nearly as widely used as those
above - especially in automotive type applications. Most synthetics on the
market will use a single PAO basestock combined with an adequate additive
package to provide a medium quality synthetic lubricant. However, PAO
basestocks are not all the same. Their final lubricating characteristics depend
on the chemical reactions used to create them.
Premium quality synthetics will blend more than one "species" of PAO and/or
will blend these PAO basestocks with a certain amount of diester or
polyolester in order to create a basestock which combines all of the relative
benefits of these different basestocks.
This requires a great deal of experience and expertise. As a result, such
basestock blending is rare within the synthetic lubricants industry and only
done by very experienced companies. In addition, although such blending
creates extremely high quality synthetic oils, they don't come cheap. You get
what you pay for! Or do you?
You can find downloadable technical data on oils here:
http://www.opieoils.co.uk"

 

Yes, I heard about that. Caveat Emptor, eh?

 

Now that's just hilarious!

 

Much like the unsupported beliefs that engines will last longer when synthetic oils are used, the old stories that synthetics cause leaks are also pretty much hearsay. There's very little evidence to support the leakage stories on modern cars and none (AFAIK) on Jaguars.

 

He's not saying that synthetic oil causes leaks. He's saying that synthetic oil can expose existing leaks. That is indeed a true statement....

 

From personal experience and from a perspective of wanting to do the best for my land Rovers (no experience with Jaguars) I put fully synth in the engine of one and it started to 'mist' around all the seals where it had been dry previously and the oil consumption went up noticeably.

When I changed back to more mundane oil the oil consumption reduced and the misting disappeared.

 

Then I bow to the voices of experience - I've never had such a problem.

I guess this tendency is just one more reason NOT to use synthetics.

 

I agree - I reckon that we are being conned by the oil & car industries into buying ever more expensive oils when, imo, we would be better using decent quality dyno oil and doing oil changes more often than the car factories recommendations.

The oil industry wants more money for it's raw materials and the car industry wants us to believe that their new cars need much less servicing therefore cheaper to run.

 

As jag state 10K miles with dino oil you don't even need to do more frequent changes. But do use a proper filter.

 

There's little difference between the gasoline oil, spark plug and filter people and the beer or soft drink manufacturers- they want to make us believe that their product is superior and have the scantily clad girls in their adverts to prove it.

Jag is quite clear about their recommendations for oil. It seems that many owners follow them to the letter with excellent results. I see no reasons to second guess them with either fancy oil or abbreviated change intervals.

However- the owner's happiness is paramount so he/she should use whatever induces a state of euphoria.