Both a 0W-30 and 5W-30 are what is known as multi-viscosity motor oils. They are often confusing to people however, in that they are both SAE 30 motor oils at your engine's operating temperature. The 0w and 5w, or for that matter, 10w and 15w, referr to the oil's performance from the Cold Crank Simulator (CCS) and Mini-Rotary Viscometer (MRV) tests. Basicly how easy the oil will allow your engine to turn over when it is cold.
However, again at operating temperature, both the 0w-30 and the 5w-30 are SAE 30 motor oils. So no, the 0w-30 is not thinner than the 5w-30.
No, AMSOIL motor oils are motor oils. It is not an additive that you add to your current oil, but is a replacement for your current motor oil. Because it is created in the lab, it does not contain the impurities that petroleum motor oils do. This allows it to do a better job of protecting your engine, and does so for a much longer time than petroleum oils do.
Because AMSOIL does not contain the impurities of petroleum oils, it can last up to 25,000 miles with the Signature Series oil. AMSOIL may be good for even longer intervals if monitored with oil analysis.
In regards to aftermarket oil additives, please do not fall for them. Even if you do not decide to use our oils, don't waste your money on aftermarket additives. Have a look at this article for more information.
Keep in mind that there is a difference between price and cost. If you are looking at price (as in price per quart) then yes, AMSOIL is often priced higher than regular petroleum oil. However, what one needs to keep in mind is the actual cost of the product. Since AMSOIL is good for 25,000 miles or 1 year, it will last over eight times longer than the "bargain" oils. So if you do the calculations, AMSOIL will end up costing less than the oil you are using now. Add on the fact that AMSOIL synthetic lubricants protect your engine better than anything else on the market, AMSOIL is the better deal.
If you would like to see the calculations, send us a e-mail, or register to view the calculations and see for yourself!