Dexron III is what the 1545 Allison in an A3 was designed to take. Don't use any other Dexrons as the newer Dexron's are not compatible with the Allison's seals. You can also use Castrol TranSynd, which is an excellent fluid, but very expensive. I would go with the Dexron III. It is very difficult to get all of the motor oil out of the transmission, cooler and torque converter. Using the repetitive drain and flush method works best in my opinion. Perhaps you can get some cheap Dexron III compatible fluid from NAPA in a 5-gallon pail and change out the fluid before you pull your old transmission. Then you are all set for the Dexron III in the new transmission. There are a lot of posts on SS about the drain and fill method of switching from MO.