Howdy,
Without seeking 'over the top' answers or voluminous discussions about;
• which Car
• which brand of Oil
• Mineral or Synthetic ...etc;

Can someone (who has read-up and become enlightened) please advise (plain terms) what are the specific differences between the following grades;

1. 5W-30 2. 10W-40

(I've always assumed its about - an engines operating environment & ambient temperature)

But I could be wrong...

 

This could turn into another debacle despite your (and my) best intentions. Hopefully not.

The numbers are simply a measurement of the oil's viscosity and NOTHING else. Viscosity is determined by the velocity of a fluid as it runs through a standardized orifice at a standardized temperature. A low rating number means that it is 'thinner' than a high rating number at the same test temperature. Motor was formerly manufactured and marketed with a single viscosity rating- 10, 20, 30 etc. and that's all there was to it. One oil was required for winter use, and another for summer, possibly with yet another required for spring or autumn. These oils are still available for very specialized use but are not suitable for modern cars used year-round in varying ambient temperatures.

Today's oils are called multigrade as they exhibit the properties of two different ratings when tested at two different temperatures. A 10W30, for example acts like a 10 weight oil when it's cold and a 30 weight when hot. The W in the rating designates 'winter' and not 'weight'.

As to which oil viscosity an OEM chooses for an engine, there's very little to be gained in second guessing or imaging that there's a conspiracy theory afoot. Usually a chart is provided which shows the ambient temperatures that the car may be exposed to, and which grades of oil are suitable. It's a pretty simple process to determine which is most suitable and not worth agonizing over. Hope this helps.

 

Thank you Mickey,
So, to put specifics to your information... all we need now are definitions (º C ) for the terms; Cold and Hot

 

Cold measuring temp is 0*F, hot is 212*F. SAE J300, first written in 1911, revised substantially in 1952 refers.

 

Just to add: simply, on cold start (0 or below) the lighter weight (0w &5w will get to your bearings,ect. faster and cause less wear than (30w &40w. On the other end (30 &40) the heavier weight protects from heat (thinning) so the parts don't contact.
Synthetic products (man made)have all the same size molecules so when you have a 30 weight you actually have a 30 weight, oil from crude has uneven sized molecules and can vary, this is why synthetic doesn't wear down as fast cause there isn't smaller molecules mixed in to disappear.simply HaHa

 

Gentlemen,
Thank you.... I feel enlightened now.

In conclusion and to my own application in a Sub-tropical / Temperate Climate
where; Winter => 40-75º F and Summer => 60-100º F, it would seem
that use of 10W-40 is more appropriate and preferred over 5W-30 ?

 

If that agrees with the info in your owners manual, then you're all set.

 

Mikey,
As you know the X-Type Owners Manual (2.5L petrol) says;

SAE viscosity rating - For climates
ranging from -30°C (-22°F) to +50°C
(+122°F), the following oil viscosities may
be used:
0W-30
5W-30 (preferred)
0W-40
5W-40

Given I am nowhere near the Colder end of the above range - I feel the 10W-40... is a better fit

What do you think ?

I am thinking Castrol 'Magnatec' Synthetic Oil

 

I would say that you've got nothing to gain by using an oil viscosity not mentioned by the OEM. The engines have proven themselves to live long and happy lives on any of the viscosities specified by Jaguar- and without the requirement for expensive synthetics to boot.

In the end, use whatever makes YOU happy- the car probably couldn't care less. I use 5W30 dino year round, our climate varies from -30*C to +35C.

 

I am happy having had this discussion and with selection of Magnatec 10W-40.
and I thank you Mickey - for your contributions and debate.

Now, all I need do to complete this exercise is to quantify an appropriate Engine Coolant...? As you might guess - an anti-freeze component is not of concern here - but, anti-boil & anti-corrosion properties certainly are.

 

Please keep in mind that when you are dealing with conventional oil, the thinner oils also do not form as thick of a layer on the surface which they are coating. This can be good, this can be bad. In older cars, the clearances between metal parts was significantly more. So, you needed a thicker oil to fill this gap and provide a seal between the parts. As manufacturing has improved over the years, they have been able to reduce these clearances down some to get more precise components out, therefore allowing for more power to be produced. But, because the clearances are smaller, you need a thinner oil to fill these gaps as the thicker oils are "too big" to fit in these clearances. So, what you would ultimately end up with if you used too heavy of oil is metal wearing on metal. As you can imagine, that is not good. Hence why most modern day cars use very thin oils (also helps reduce the load on the engine from the oil pump since a thinner oil is easier to move around and therefore does not result in as much parasitic losses).

If you really want to get to know about oil, what brands are better than others, whether to use conventional oil, full synthethic, or a blend, go check out - Bob is the Oil Guy. This is a guy that spends his own money analyzing oil and calls things as he sees them. I think you might be surprised as to what you can find on his site.