JMH,
The 0w-30/5w-30/10w-30 are now all ACEA "A3/B4" rated, although all the spec sheets on the website haven't been changed yet. If you look at the labels on the quart and gallon containers you'll see the newest ratings. I believe the new spec sheet for the 10w-30 has been posted on the Amsoil website - the other two oils will basically meet those same specs.
The regular 5w-30 and 10w-30 are very similar oils that use the same additive chemstry - I'd consider those to be interchangable. The S2000 and S3000 are also similar basestock blends, but the additive chemistry in the 0w-30 is optimized for gas engine use, while the Series 3000, 5w-30 is a fuel efficient, HD diesel oil. You can use the S3000 in gas engines, it's ACEA A3/B3 rated, but there is no particular reason to do so.
The only one of these that may give you better performance and fuel efficiency is the S2000, 0w-30, since you are already running a friction modified 5w-30. The 0w-30 uses a different basestock blend and there are some differences in the additive chemistry which they are very tight lipped about. I've seen very similar wear rates with all these oils, but TBN retention is a bit better with the S2000 and S3000. So if you are running change intervals longer than 15,000 miles those would be preferred.
When the S2000, 20w-50 racing oil first came out in 1995, I ran some in my 1990 Audi 100 which had about 100k miles on it at that time. On long highway trips @ 70-75 mph, I was actually getting the same gas mileage as I did with their regular 10w-30 synthetic. You would normally expect to lose 3%-4% in fuel efficiency going from a 10w-30 to a 20w-50. That convinced me it was a different formulation with more effective friction modifiers.
TooSlick