They are essentially the same the oil. AMSOIL has upgraded their whole line of 2 cycle oils.
The ATC 100:1 has be around over 25 years and due to new technoliges in 2-cycle vehicles
and equipement AMSOIL saw the need to rename and label the 2-cycle line of oils to help customers
select the most optimal 2-cycle oil for the their particular needs. The ATC and ATP are for
air cooled engines and are mixed 100:1, the ATC will be discontinued once supplies are depleated.
You may have noticed or heard that there is now an ATO which is a 100:1 pre mix just for outboard
engines. If you have any further questions feel free to give me a call at 1-800-481-2649 I am usually
avaialabe in the mornings from 9:30 to 12:00 and again in the evening from 5 to 8:30 PM, evenings
being better.

Ralph Graul
AMSOIL Direct Dealer