I just bought a A66 and it has ok power, it will go up a hill in low but even on the level in high it falls on it's face and moves slower than when in low. It has atf in the Trans and I an going to drain it and switch to 10w30 per the dealers advice. Anything else I could check or do?
I would find the manual for it and find out what is supposed to be in that transmission.
I have a A62 and I use the hydropower oil from Newholland, wich is Sae 10w, and the right oil for the allison transmission. Is your oil level okay? because my does that when its low on oil. you can also try to cet it up in working temperature before shifting to high gear.