[glhs#116]

As a vastly general rule of thumb I was always advised to steer clear of oils with large ranges (e.g. the OP's 5W-50). The decode on that, of course is "acts like a 5 weight cold and acts like a 50 weight hot". In other words. it thins out as it heats up but much more slowly than a "straight" oil. Now what I've always been told is done chemically in order to get this behaviour is you start with a base stock that is essentially "the lower weight" (in other words, a 5 weight base stock) and then you add more an more "thickeners" to get the larger and larger spread (e.g. from 5W-20 to 5W-30 to 5W-40 to 5W-50...).

Now, the tradeoff -- as I understand it -- is that in an ideal world we'd love an oil with the highest spread as this would give the most consistent viscosity to the engine. However, in the real world those thickeners break down and the more of them there are the more the oil is changing over its lifetime. I've always understood that one of the best real-world compromises is basically a 5W30 or a 10W30.

Now, also in the real world all sorts of specific blends may have all sorts of specific strengths and weaknesses. For what it's worth I have stock clearances and run the modern Shell 5W-30 (it's synthetic) in the "AG" variety (all the modern oils come in like "Ford", "GM", "Mercedes" etc flavours and "AG" is the "GM" flavour).

Sam