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MG MGA - Thicker oil

An oil cooler reduces the temperature of the oil. The oil, being hot is in a more fuild state before it enters the cooler ( my assumption ) . Now the question....when the oil leave the oil cooler has it return to its origional viscosity, as it was in the origional container.
Gordon Harrison

If I am not mistaken, use of multi-grade oil prevents the oil from being thicker as it leaves the cooler....It is thinner at the lower temperature, and thicker at the higher temperature...So the point of cooling the oil, is to keep it at the lowest possible viscosity number...At least that is my understanding.
Edward Wesson 60MGA

"it is thinner at the lower temperature, and thicker at the higher temperature,
Don't think so Ed. Multigrade oils do reduce the temperature effect on viscosity, but they don't reverse it!
So, Gord No. In the original container it was cool, in the enine and ex the cooler, it is hotter and thinner.

Art Pearse

Up to an optimum value the oil needs to be hot and thin to circulate around the engine that is why any oil cooler should always be used with a thermostatic valve so as not to cool the oil below a certain temperature. You only have to look at some modern synthetic oils to see how thin they are but their use with old engines is another matter again.
J H Cole

I know nothing about this thread, but I picked up on John's comment about these thin oils in our old engines. We discussed this a long time ago and I am aware that many B series engines are run regularly on fully synthetics with low numbers. I ran mine on a 5-20 for a full season without any undue problem, although it perhaps wept a bit
more at the joints compared to say a 20-50.

Steve Gyles

Maybe my statements were an over simplification...But , I stand by my theory, that the premise is to keep the oil flowing at the optimum temperature and viscosity.....
A direct quote from Wikipedia (not necessarily my regular source, but handy, here):

"The SAE designation for multi-grade oils includes two viscosity grades; for example, 10W-30 designates a common multi-grade oil. The first number '10W' is the viscosity of the oil at cold temperature and the second number is the viscosity at 100 C. The two numbers used are individually defined by SAE J300 for single-grade oils."

So 20W (W for Winter viscosity, not weight),50, has a viscosity of 20 when cold and 50 at 100 centigrade....

Once it passes through the cooler, on its way back to the engine, it will drop down from the 50 viscosity #....

Obviously, it will not go all the way back to a viscosity of 20, but it will also no longer be 50...(IMHO)...

Edward Wesson 60MGA

All good stuff Edward, but, from my simple non-scientific experience, - multi grade, single grade, mineral, synthetic, in cars, motor bikes, boats, whatever - when it's hot it's thinner than when it's cold! We all know that surely? Just try using a vacuum extractor pump for an oil change when the oil is cold (almost impossible) compared to when it's hot (easy)). Now, whether "thick" and "thin" are anything to do with viscosity numbers I don't know, but oil definitely flows easier, ie. seems "thinner", when hot.
Anyone scientific out there care to correct my non- scientific ignorance!?
Bruce Mayo

You are absolutely right Bruce.
I think the numbers 20, 50 etc are just grades, not actual numerical viscosities.
Art Pearse

This makes an interesting read:

Steve Gyles

There is some confused thinking here guys. The 'SAE' scale is one of many recognised ways of indicating / measuring viscosity or how thick or how resistant a liquid is to shear, so the numbers do refer to a precise viscosity value.
If you have the old 'straight' oils say SAE30, it will have the viscosity of an SAE30 oil when its cold and be very, very much thinner when hot. Not so good for the protection of a hot engine...
Now a multigrade with its additives, like say a W20/50; when cold it will have the viscosity of a cold 'straight' SAE20 oil (good for a cold start) and when hot, although actually thinner, it will then have the same viscosity as a hot 'straight' SAE50 oil - with better protection for a hot engine. So you get the best of both scenarios.
Hope this explains the two numbers.
So you can see these modern oils have a flatter viscosity curve - so when it comes out of the cooler it probably won't really be that much thicker.
Hope this helps
P N Tipping

Steve, there's a lot in the article but for me, if I've got it right, owners using traditional multi grades such as 20W 50 should replace their oil frequently because of the degradation of the viscosity improving additives. (Most owners would probably do this anyway). As the article says an old 20W 50 might in reality be a 20W 20! The article seems to lean towards synthetic oils that don't need the additives and probably can go longer between changes. There is still the issue of the oil film at cold start up whereby a 20w mineral oil might be better than 0W,5W or 10W semi or fully synthetic oil for an older engine.
J H Cole

This thread was discussed between 10/05/2014 and 12/05/2014

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