That actual definition is that the oil has a viscosity equivalent to a straight zero viscosity
oil at a specific temperature.

Now, a 0W40 is actually a 0 weight oil that does not thin out as much
and maintains a viscosity equivalent to a 40 weight oil at the higher rating
temperature.

The problem is that to do a spread of 40 points requires a lot of viscosity
index improvers. These shear when in tight confines resulting in going out of
rating and sludge. This is known as high termperature shear or HTS.

A 25 point spread is easier to accomplish. 15W40 is a 25 point spread.
And it starts out as a 15 weight oil. The pour point of Shell Rotella conventional
15W40 is down around -35*C. More than adequate for the coldest temperatures
seen by just about all readers on JF and especially down under.

Don't forget that the oil galleries are usually still mostly full. Oil is non-compressible.
The laws of hydraulics say that if you develop pressure, the oil *will* be there.

A thicker base weight is also more likely to remain available as a protective film
at first fire before the engine has even rotated a quarter turn. Well before the
oil pump could be hoped to deliver any oil at any pressure.

Finally, there is one thing that conventional oils do better than synthetics ...
carry away heat.

Curries did an experiment ... synthetic differential lubes were measured to run
cooler ... but what about the bearings and gears? The actual parts the lubes
were supposed to be protecting were running hotter! The synthetic was
not picking up the the heat and therefore ran cooler at the expense of the
parts needing the cooling!

edit:

currently on 0W40 Shell Rotella because I *do* drive in -35*C temps,
but going back to 15W40 because the engine does not crank any
better. i'm more comfortable with the 15W40 coming from 20W50
Castrol GTX.

 

Hi Bro

Down my way its the opposite, with 0W-40 being the cheapest, as its probably not so popular.

When I started driving, everyone was using 20/50 but as 'The Wizard' has pointed out things have now moved on.

I'm not too bothered about the Price, as I can get a good discount on Top of the Range Stuff of any Vis.

 

Thank You for that Plums!

Very, Very, Informative, as I too was wondering if the 0W/40 clung onto the moving parts, at those times when the engine isn't running.

Which kind of brings things full Circle, 'The Wizard' was right all along! so as for me I will take that advice and go with 15W 40W.

 

My Iron Lady gets 20W50 mineral oil (non synthetic) as the entire service history has exactly that kind of oil... So why change something the engine was made for?

 

I am not an engineer but some of the posters at "bobistheoilguy.com" are and you will get a better understanding of oil at that website... Its a great forum including topics about oil, coolant, tires, hydraulics, etc... I am not sure in the UK which oil you guys can purchase but German Castrol 0W40 is also a recommended oil based on used oil analysis.

 

Hi Daim

This Car has always had Synthetic, so that is what I am going to stay with.

 

While I'm sure this guy knows what he's talking about, I think I would rather listen to someone who's had a lifetime of experience with this 'Marque'

What 'The Wizard' doesn't know, you don't need to know, at least as far as I am concerned.

 

All my knowledge is from having over 40 cars in my lifetime and from reading and talking to experts in the car industry. Like I have previously posted, check out the website... I also know a few of the top classic car collectors in the USA. I'm sure just about any 40 or 50 weight conventional or synthetic oil would perform well in our Jags. I have used mostly Shell Rotella and VR1 in my Muscle cars and a few classics but they were much older vehicles then my 90 XJS.

 

There is some good information from some participants
at bobistheoilguy. Doug Hillary out of AU comes to mind.

There is also a lot of regurgitated gospel.

For example, the famous Dr. Hass theory on low viscosity
oil. Well, the good Dr. is a DENTIST! Good guy to see if
you have a tooth ache. Might even know something about oil.
But certainly no professional qualification in the area.

Yet, people keep on looking at the "Dr" part and presume he is
some kind of engineer or scientist in the area as they punt
the idea one more time into the interwebs.

Better sites exist such as certain lubrication consultancies,
and the site for the Journal of Machinery and Lubrication.

Learning takes critical thought about the idea being presented
and the presenter.

It's easy to read the internet, but much harder to sift the gems
out of the chaff.


++

 

I've had the weirdest stuff happen to my older cars after changing the oil type and viscosity.

Started with my Volvo 960 3.0l I6. Factory states 5W40 partsynthetic oil. I filled 5W40 fullsynt. into it. Result: it started to weep around the head seals. My C30 got 0W30 from day one. My dealer changed somewhere along the lines to 5W30. The result: oil consumption went up. I made sure they went back to 0W30 and it went down to zilch again.

Leaks, consumption and blocks... Keep with that, what the engine has had last and longest and all is good That is my opinion as a normal person who doesn't expertise on oils...

 

Yes Daim I Agree!

According to the Handbook which I dug out This Morning 15W-40 should suit my engine fine and so I see no reason to experiment with anything else.

 

I agree with you 100% on your statement.


If you are going to use Rotella 15W40, it is an excellent conventional oil and I have used it for years in a lot of cars. Use it with confidence.