From what I've been taught in automotive maintenance classes, and from picking the head of a friendly ASE master mechanic (Dad...) the viscosity is mostly just to accommodate different temperature ranges.
Some manufacturers of engines have shifted what their engine requires for said ranges, usually down in viscosity, seeking better mileage, but it still varies by temperature.

The thing that makes a difference between oil for diesel cars versus gasoline ones is the additive package in the oil.
If you look at the back of the bottles, there should be a circular logo with a two letter rating in the ring around it. A letter code such as CF or CH is for diesel engines. (as far as I've ever been able to tell, the C first letter stands for compression ignition)
On the other hand, gas engines are covered by S* oils, which were down to SM or so, the last time I looked at one.

I don't know why, but I have only typically found both ratings on 15-40w oil, but you can occasionally find C rated oils in other weights.


That all being said, who knows just how well some of the new synthetic oils hold up?
M1 doesn't seem to rate their oil with the same rating scheme, and I've never actually seen a bottle of Amsoil, so I can't say there.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...