I'm not advocating that 0w-40 is the solution to all engines as they vary so much due to modding and set-up.

What I am illustrating here is that an sae 40 (be it 0w-40, 5w-40, 10w-40 or 15w-40) is still an sae 40 and at 100degC thins to 14cst.

I am also stating that an sae 40 is good for an oil temp of 120degC and the switch to an sae 50 within this temperature range is wasting power at the wheels.

Sure if you are seeing oil temps above 120degC then an sae 50 is certainly a better choice for a higher level of protection in extreme cases.

With regards to oil pressure, I had a long discussion with a technical expert recently and he made some valid points which were as follows.

Oil pressure is initially defined by engine design, and it doesn’t have to be high.

The mighty Cosworth DFV V-8 which dominated F1 for 15+ years was (and still is) happy with about 45PSI. The Le Mans-winning Bentleys of the 1920s were designed to run on 5-10PSI once the oil was hot.

Obviously, an engine which would always give massive oil pressure would be one built with microscopic bearing clearances, so there would be nowhere for the oil to go! 150PSI, but total seizure in a few seconds!

On the other hand, oil pressure significantly lower than the spec. minimum means that bearings ‘at the end of the queue’ for oil feed get a low flow rate, which is not desirable.

Even so, a good flow rate with a wear-resistant low-viscosity oil will improve cooling and be better for the engine than another 10PSI with a thicker oil. (….and less chance of cavitation problems.

With regards to marketing guff, I think that some of my earlier posts concerning shear stability say it all. It's not guff, it's fact. All oils shear (lose viscosity) with use and this means lower protection at higher temperatures. I've explained this before but it's probably on page 1000 by now so I'll post the explanation again.

Viscosity Index Improvers.

An oils viscosity will decrease as the engine temperature rises. Viscosity Index Improvers are added to reduce this thinning. They are a key addative in the production of multigrade oils.

VI Improvers are heat sensitive long chain, high molecular weight polymers that increase the relative viscosity of the oil at high temperatures. They work like springs, coiled at low temperatures and uncoiling at high temperatures. This makes the molecules larger (at high temps) which increases internal resistance within the thinning oil. They in effect "fight back" against the viscosity loss in the oil.

"Shearing"

The long chain molecules in VI Improvers are prone to "shearing" with use which reduces their ability to prevent the oil from losing viscosity. This "shearing" occurs when shear stress ruptures the long chain molecules and converts them to shorter, lower weight molecules. The shorter, lower weight molecules offer less resistance to flow and their ability to maintain viscosity is reduced.

This shearing not only reduces the viscosity of the oil but can cause piston ring sticking (due to deposits), increased oil consumption and increased engine wear.

Like basestock quality, VI Improvers also vary in quality. The best quality ones are normally found in synthetic oils (Group IV - PAO / Group V - Esters) and it is important to understand that the less of these in the oil the better the oil will stay in grade.

Which oils require more VI Improvers?

There are two scenarios where large amounts of these polymers are required as a rule.

Firstly in "wide viscosity" multigrades. By this I mean that the difference between the lower "W" number and the higher number is large for example 5w-50 (diff 45) and 10w-60 (diff 50) are what is termed as "wide viscosity" oils.

Narrow viscosity oils like 0w-30 (diff 30) or 5w-40 (diff 35) require far less VI Improvers and therefore are less prone to "shearing".

Secondly, mineral and hydrocracked (petroleum synthetic oils) require more VI Improvers than proper PAO/Ester (Group IV or V) synthetic oils as they are less thermally stable to begin with and this is due to the non-uniform molecules in petroleum oils as opposed to the uniformity of synthetics built in laboratories by chemists.

It is a fact that some synthetics require little or no VI Improvers to work as a multigrade due to their superior thermal stability.

At the end of the day it's your car and your money but without the facts I can't see how you can possibly make an educated decision.

Cheers
Simon