The ally can take synthetic. Castrol makes the synthetic Transynd specifically for those trannys. AMSOIL does make the Torque Drive which is an exact replica of the transynd. These meet allison's TES-295 spec. They also qualify as meeting GM's dex 3. GM has now gone to dex 6. Our universal ATF meets this newer spec. If I was going to switch it out, I'd lean towards the ATF. Newer more advanced fluid and if still under warranty, meets what GM wants in there.
As far as mixing or flushing, flushing is the better option. You get more out of what you put in there, but some do mix...just drop the pan and refill. Gives you about a 50/50 mix. You wouldn't want to run this near as long, but it works.

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Heath, is there a way to flush the trans by myself? like in my own shop?

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