Oil - does grade trump specification?

[zebedee]

If a car needs SAE 5W-30 to spec WSS-M29C913B but says if you can't find it you can use SAE10W40 meeting ACES A3/B3 and I have the choice of the latter or and SAE 5W-30 to a different spec (I think something like A3/B5) which one should I use? As I see it one is the wrong grade, but is in (admittedly a less than ideal) spec (manual says may result in longer cranking, reduced performance and economy), the other is the right grade but not the right spec.

I need to top up to get home today, when I will then sort it with the right stuff.

[zebedee]

Mave said:

If you have an oil that is listed as an alternative, I'd use that rather than one that isn't.

yep did that, and got a really good fuel consumption with it too! It gets serviced next week so will be getting some proper Ford-grade Castrol Magnatec.

[zebedee]

I did a bit of reading up yesterday, but I was still confused. It seems A1/A3/A5 is something to do with the way the oil performs under sheer stresses - so A1 is better for engines with low stress and give better economy, A3 for higher stress but A5 kind of combines both, but they all say you need to make sure the oil is suitable for the type of engine. My manual says nothing about A5 so I'd be wary of that one, but I used an A3 10w-40 in the end

[zebedee]

Mave said:

In that case using a thinner oil (5w-30) which is less capable of handling "stress" would have been the wrong choice.

this is why I got confused, apparently the thin/thickness thing has nothing to do with stress, it is to do with viscosity at certain temperatures. So you could have a 5w-30 A3 and a 10w-40 A1 and the 5w30 performs better under stress.

Makes sense, some motorsport oils are 0w, but hugely stressed.