I specifically remember "synthetic 5W-30" mentioned regarding recommendations for my dad's 1996 Regal GS regarding better low temperature performance.
Of course I agree that the term these days is more about product differentiation than about any truly meaningful term. That wouldn't have been true 15 years ago, when it pretty much meant a group IV/V combination.
There are certainly recommendations although not necessarily outright requirements. There's the current recommendation for Subaru turbo vehicles:
[quote]
http://techinfo.subaru.com/proxy/69530/p...314ASTIS_19.pdf
You should use synthetic engine oil that meets the same requirements given for conventional engine oil. When using synthetic oil, you must use oil of the same classification, viscosity and grade shown in this Owner’s Manual. Refer to “Engine oil” F12-5. Also, you must follow the oil and filter changing intervals shown in the Warranty and Maintenance booklet.
http://techinfo.subaru.com/proxy/69530/p...314ASTIS_20.pdf
5W-30 synthetic oil is the required oil for optimum engine performance and protection. Conventional oil may be used if synthetic oil is unavailable.
*: 10W-30, 10W-40, 5W-30 or 5W-40 conventional oil may be used if replenishment is needed but should be changed to 5W-30 synthetic oil at the next oil change.