The reason for imposing a one year time limit is to account for variations in driving conditions. For example, if you only drive 10,000-12,000 miles annually, but it's all short trips, you engine oil will degrade as much as if you did 20,000-25,000 miles of pure highway driving. So it is a very good idea to change Amsoil at least once a year. Having said that, I occasionally run the same batch of Amsoil for two full years in my tacoma pickup - typically about 15k-20k miles - and it does just fine. However, I always do an oil analysis after one year; have over twenty years of experience as an Amsoil dealer and know exactly what I'm doing. I don't recommend going over a year with Amsoil unless you are doing oil analysis and working with someone like Terry Dyson that can give you expert advice ....
In addition to annual changes, Amsoil also recommends changing their oil filter and topping off the crankcase after six months or 12,500 miles. In my opinion, other brands of filters should be changed every 5000-6000 miles or six months, since most cheap filters don't have much dirt holding capacity. Note: The Mobil 1 oil filters should also last for 10k-12k miles, but this is also a $12.00 dollar, ultra premium filter.
This six month filter change helps keep the oil clean for a full year and introduces 0.5 to 1.0 qts of fresh oil. The fresh oil dilutes any contaminents and helps replenish the additive package of the oil in the crankcase. It is extremely important that you change the Amsoil filter and topoff after six months, particularly if you do lots of short trip driving. The resistance to flow inside the filter also starts to degrade the minute you start the engine for the first time and it starts to catch insoluble particles.
Tooslick
[ February 15, 2004, 08:00 AM: Message edited by: rugerman1 ]