[T3] Synthetic Oil?

[Jim Adney]

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.

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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