[T3] Synthetic Oil?

William Jahn willjahn975 at gmail.com
Tue Oct 27 13:27:12 PDT 2020

 I read the entire post.
So far no one has really gone into detail why synthetic oils in air cooled
engines don't transfer heat from the heads as well as mineral based oil.
The oil system on the VW air cooled engines is fairly easy to understand. I
can't say why the case will run cooler with syn oil and the heads hotter.
Best guess is less friction at the bearings where most of the oil moves
because of the slickness of syn oil. The case has its own return valve and
there is the bypass valve for the cooler. Once the engine is hot it's all
maintaining pressure and flow and cooling. The heads only get oil through
lifters and pushrods for the rockers and valve stems and drain back via the
push rod tubes. I imagine it's possible the syn oil flows too fast through
the heads to pick up the heat, not so much because of the viscosity ,
rather the slickness of syn oil allowing it to move too fast. This is where
it gets tricky because this would also apply to the cooler yet if you
factor in the less friction at the bearings it may balance out.

 This is just a wild guess on my part since I really don't know. What I do
know is I will continue to use mineral based oil even if my engine was new
and broken in well with no build up to be washed away.  Without a filter
even if syn was better can't justify the price difference over mineral oil.
Besides VW never recommended it and I prefer to stick with that.

On Tue, Oct 27, 2020 at 10:23 AM Jim Adney <jadney at vwtype3.org> wrote:

> On 26 Oct 2020 at 12:47, William Jahn wrote:
> > I also went on a site from a fellow who has decades of building VW air
> > cooled engines and he said never use synthetic oil in air cooled because
> > the case temps will read normal yet the head temps will be much higher
> > because the synthetic oil does not carry away the heat fast enough.
> What he's repeating is Gene Berg's claim that synthetic oil doesn't carry
> heat away as well as mineral oil. I've heard this claim before, but I've
> not
> heard anyone state why this might be true. It seems to me that it could
> only
> be true if the specific heat of the synthetic was significantly lower than
> that of
> mineral oil. That's certainly possible, but it seems unlikely.
> What I DO know is that there's almost nothing that has a specific heat as
> high as water. Water is an excellent coolant; nothing else comes close.
> The specific heat of water is 1.0 cal/gm C.
> For mineral oil I find numbers from 0.4 to 0.53, where it seems to go up
> with
> increasing temperature. One source gives a value of 0.60.
> For synthetic oil I find numbers around 0.57, at moderate temps, but no
> data
> for higher temps.
> The best info I could find was a Brazilian paper that did a comparison
> between different families of oil after they were each degraded by
> heating, to
> mimic the aging that would occur during use in a hot engine. They tested
> automotive oils in 4 families: mineral oil with additives, mineral oil
> without
> additives, semi-synthetic, and synthetic. Note that they were writing in
> English as their second (or third) language, which makes the paper hard to
> read in places.
> Look at tables 4 & 5 here:
> https://is.gd/GQGDnH
> You'll see that these numbers look different, because they are given in
> Joules/gm C deg instead of cal/gm C deg. You can convert to the other
> units
> by dividing by 4.18.
> Especially interesting are Figures 3 thru 6, which show INCREASING
> specific heat with increasing temperature for almost all of these oils.
> That's a
> great thing for any engine.
> In all cases the synthetic was slightly lower than the mineral, but the
> difference strikes me as insignificant. It's possible that the densities
> of the
> mineral and synthetic oils are quite different. If so, that would change
> everything, since we are generally interested in how much heat some
> VOLUME of oil can carry away. I have not managed to find a good
> comparison of the densities of mineral and synthetic engine oils.
> Finally, here's a paper written about lubricants for army tank diesel
> engines.
> They talk about thermal conductivity as well as specific heat. Personally,
> I
> suspect that conductivity is much less important to us than heat capacity,
> and this paper makes no attempt to make a judgment on this.
> My personal feeling is that synthetics are probably just as good as
> mineral
> oils in heat transfer, but they may tend to loosen up old deposits in an
> engine that's been run for years on mineral oil. The released dirt can
> damage an engine, especially one that does not have a filter, like ours.
> A similar thing happens to diesels that are converted to bio-diesel. The
> bio
> diesel is a great solvent and loosens up all kinds of sediment from the
> fuel
> tank and supply lines. Recent conversions to bio will find their fuel
> filters
> clogging and some will blame the quality of the bio fuel. Many will throw
> up
> their hands and change back to dino diesel. In most cases, it takes 2-3
> fuel
> filter replacements to get the system clean, then the bio diesel runs fine.
> Sorry this went on so long. My regards to you if you managed to read to
> the
> end.
> --
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
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