R2R,
I like to run 5000-6000 miles on petroleum oil before putting Amsoil in for the first time, unless the particular engine came with synthetic from the factory, as is the case with some German cars. I'd suggest doing two, 3000 mile oil changes using any decent petroleum oil and then putting in Amsoil. Either their 5w-30 or 10w-30 will work fine in this application.
In the case of automatic transmissions, I like to make sure the unit is functioning normally and isn't defective before putting in Amsoil. I'd go 20k-30k/1 year on the factory fluid and then change to the synthetic ATF. If possible do a total change and drain the torque convertor as well as the oil pan.
TooSlick