I've got a question regarding this discussion. Why do so many of you use oil with such a wide viscosity range, i.e. 5w40, 0w40, 5w50, etc?
I've done quite a bit of research on the subject because it interests me, and from what I've found, you are much better off using something like a 10w30 or a 15w40, particularly in a hot running turbo motor.
Even with synthetic base stocks, it is necessary to use large quantities of viscosity index improvers to give the oil such a wide viscosity range and this will cause the oil to break down more readily under thermal and shear stress.
A 10w30 or even a 15w40 synthetic will flow down to very, very low temperatures so you are not really gaining anything by using a 5wXX or a 0wXX oil, but you are giving up some high temperature stability.

Shimster,

Any conventional oil, and hence semi-synthetic, will contain impurities such as wax. At low temperatures the wax forms a crystaline structure which will thicken the oil and prevent it from flowing freely around the engine. That's one reason why I always use a synthetic oil in any car I own.

harrydog,

It's very difficult to get any information about which oils use viscosity improvers or whether they are PAO or ester base stocks. I'm using Motul 10W-40 no viscosity improvers and an ester base stock. Most other brands keep this info under their hat.

Andrew,
That's good to know as I believe the ester base stocks are more stable at high temperatures than are the PAO's. Sounds like you've made a good choice.