Why would they recommend the best for the car? Manufacturers don't want the cars to go on too long. Also they need to recommend things that are easily available all over the world and you can get a 10w-40 pretty much everywhere, but it used to be much harder to find a 10w-40.
Cheers
Tim
Yes, all OEM accross the world manafacture so their cars don't 'go on too long'
Providing they reccomend fluids which are readily available through the dealer network, there's no problem.
25years ago when the G40 was being developed oil technology wasn't what it is now, however the additional additives that 5W-30 has over 10W-40 will mean it breaks down in high heat/high shear situation and can cause sludging before the 10W-40 does (all things being even), possibly why VW chose a 10W-40. The delta between the 'W-winter' figure and '-XX' figure is usually a good indicator to quantity of additives in the oil to achieve this range.
As with anything once additives break down, oil performance (viscosity at operating temp) can degrade, which is generally why most race engines will run on straight weight oil or a small delta between the two quoted figures- Royal Purple's range shows this well (0W-5 available!).
No one (including Opie) can be 100% confident unless running a full durability programme (as per OEM) on the intended oil, including climatic testing. However in the real world I doubt you'd see any degredation in oil quality between the two oils, given that most G40 owners change the oil every 3,000miles or so.
Pete
