Just a very simple question from a very simple guy, what oil is specified for the latest DPF equipped diesels (now that C! has been dropped)? Should we just be using that? Drives:
2010 FFRR TdV8 Vogue - Stornoway Grey
2010 FFRR TdV8 Vogue SE - Zermatt Silver
1998 BMW E36 M3 GTII

The answer is in my OP on the matter, the best oil is already available: 5W40 C4 ULS, but folks are too busy arguing over the unproven alternatives and the demand remains low, so the price is high. I for one would never put C3/C4 into my Euro 6 car after carefully comparing the ACEA documentation.
So I continue with the 5W30 C1 ULS every 2.5-3K miles. For V6 owners in particular, my best recommendation is to fit a new OEM oil pump at each timing belt service, there is no substitute for good oil pressure, no matter what the VI.  V6 Vogue - “Less is More” - less to go wrong!

Oil pressure is not the same as oil flow..... 2012 TDV8 Vogue (L322)

Did anyone say that?
However if you increase fluid pressure through an orifice, then more fluid passes, so flow must therefore also increase, yes? It’s the oil pressure which keeps the metal surfaces apart, low oil pressure means more wear. V6 Vogue - “Less is More” - less to go wrong!

If oil flow is restricted due to blocked ports (eg sludge from extended oil drain intervals) or it's viscosity is higher than OE then the oil pressure looks healthy but it may not be flowing enough to protect the bearings. Oil flow is the critical thing which is not directly measurable and only estimated by relating pressure to temp and oil viscosity with an assumed clean engine. 2012 TDV8 Vogue (L322)

What JayGee said. The first number followed by a W in multigrade is the Winter or cold performance fluidity measure & the second number After the W is the Hot performance fluidity measure. So 5w30 & 5w40 act like a 5 weight oil at low temperatures, but when hot as inside an engine at normal operating temperature, one will act like a 30 weight oil & the other like a 40 weight oil. Lower number=thinner oil.

If you want to do a simple test just get some new engine oil & some EP90 gear oil & drip one drop onto a ruler propped up at 45 degrees. The thinner engine oil will travel faster than the thick gear oil.

Multigrade oil is a modified base oil using additives (mostly polymer chains) to change the behaviour at different temperatures. Thus ideally you want the multigrade with the narrowest range that meets the standard & your application's requirements.

So a 5-30 has a narrower range than a 5-40, so will maintain its high temperature properties for longer as it ages assuming the same loads/cycles as a 5-40, but I don't believe 30 weight is ideal for high load applications. Where a heavier weight oil would be better suited to high temperature loads. Yes 5-40 will degrade its 40 rating to a lower rating over time, but you have to balance the fact it started life as a 40 weight when hot & will degrade towards a 30 weight over time. But a 30 weight "may" degrade more slowly towards a 20 weight, but will do so more quickly if it is exposed to high temperatures &/or high loads. Either way it will never be thicker than 30 weight.

Now all the above is fine on a petrol application, but throw in stupid diesel over fuelling for DPF regens which leads to oil dilution from the excess diesel that bypasses the piston rings, then that 30 weight can quickly become water weight, ask any Ingenium 2L owner! So starting with a 40 weight oil should provide a stronger oil film & higher shear resistance for longer regardless of dilution.

Obviously reducing the oil change interval to account for dilution will help too.
Interesting how oil used to be rated for 12K miles, then manufacturers started increasing these schedules to improve their Green credentials, regardless of the fact that oil never got better & the oil companies pulled a fast one & reduced the standards for "Synthetic Oil" to allow lower mineral based oil to be called Synthetic!? When its not really, but it sells more !

10w40 is perfectly suitable for use in temperate climates if you could find a C3/4 rated one for diesels. I've used it in my motorbikes forever & they have a far higher duty cycle than most cars, double the rpm's & generally a higher specific power out put, but less torque load shear than a diesel. But feeding the gearbox makes up for that as the oil is subjected to mechanical shear in there.

Strangely there is a Motul 10w50 oil for the Seat Abarth 0101 spec high performance engines !

@ Mohawk - "So 5w30 & 5w40 act like a 5 weight oil at low temperatures, but when hot as inside an engine at normal operating temperature, one will act like a 30 weight oil & the other like a 40 weight oil. Lower number=thinner oil."
The oil is still thicker at lower temps as the numbers on the w rating are not relatable to the operating temperature rating. 2012 TDV8 Vogue (L322)

The 5w rating is the same, but like all oils there is an upper & lower fluidity limit for 5 weight oils at the chosen test temperature, so a 5w30 is probably thinner than a 5w40 at low temps, but both are within the permitted range. One manufacturer to another's oil will be different but within range to state its 5w is 5 weight. So negligible differences unless you live in the Artic. Check out the ball bearing drop test videos for oil viscosity on YouTube comparing different multigrades with the same W number, but usually at very negative degree C temperatures.

But you agree the oil gets thinner as it warms up? 2012 TDV8 Vogue (L322)

Yes, Absolutely but 30 gets thinner than 40 & has a lower pressure in the same pumped system & lower shear resistance when warm.

JayGee wrote:

Mikey wrote:

Mohawk wrote:

The point being that 5w-30 Cx is & will always be thinner than 5w-40 Cx, but 5w-40

So why not use a 10w40, or a 15w40?

It's way too thick to circulate in the engine on a cold start esp in temperate climates.

Its not night & day, or say water -v- cold honey or warm honey even. 40 is just a bit more resilient than 30 at higher temperatures.

Castrol GTX hot viscosity
5w30 C3 kinematic at 100c=12
5w40 C3 kinematic at 100c=13.6
Means that the 40 is approx 13% thicker @100c than the 30.

But when very cold they are roughly the same with a 40 pour point of -45c & 30 at -42c. So according to Castrols own published figures the 5w40 does better than the 5w30 at both ends of the spectrum!

Your money, your car, your choice.

Full disclosure mine is currently running 5w30 C1, as was under full Jag warranty until last month. I just bought 10L Castrol GTX 5w40 C3 on the Amazon deals days yesterday & will be doing an oil change for its service later this month.

We will see how it feels & how it goes.

Mikey wrote:

JayGee wrote:

Mikey wrote:

Mohawk wrote:

The point being that 5w-30 Cx is & will always be thinner than 5w-40 Cx, but 5w-40

So why not use a 10w40, or a 15w40?

It's way too thick to circulate in the engine on a cold start esp in temperate climates.

How do you know a 40 isn't too thick?

Some of us are old enough to remember when most cars were running on 20W50 and acheiving high mileages without any issues, the design of the internal combustion engine hasn’t significantly changed since then.
These “watery oils” are just a cheap method towards meeting mileage/emissions targets by reducing hydraulic resistance, possibly at the expense of longevity, but why would the manufacturers care? V6 Vogue - “Less is More” - less to go wrong!

Modern engines are built to much tighter tollerances and thus can take advantage of lower viscosity oils and not need thick goop to fill the sloppy fit of parts. 2012 TDV8 Vogue (L322)