The Series 2000 engine oils are Amsoils ultra premium line of gas engine oils, specifically formulated for severe service applications. The S2000, 20w-50 really isn't intended for super long change intervals, but holds up very well for 12k-15k miles in my Audi 100. The 0w-30 is intended as a replacement for 5w-20/5w-30/10w-30 grades. I really like it for short trip or cold weather driving conditions, where it provides noticably better fuel efficiency - typically 2%-3% - than their 10w-30.
The Series 3000, 5w-30 is a low viscosity "mixed fleet" SL/CI-4 type oil for diesel/gas engines. The main application is as a fuel efficient replacement for their 15w-40 HD diesel oil in commercial diesel engines. The S3000 and S2000, 0w-30 use a similar basestock blend, but the additive chemistry of the S3000 is optimized to deal with the combustion byproducts of diesel engines. Having said that, the S3000 is also an excellent extended drain, ACEA "A3" rated lube for gas engines. In my own tests, it holds up at least as well as the 0w-30 in these applications. The S3000 is also Amsoils' most shear stable xw-30 oil. Probably the best all round low viscosity oil they are making right now.
If you have any other questions, feel free to contact me offline.
TooSlick
Dixie Synthetics