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Somewhere between 5 and 6 gallons. Use the allison 4 inch deep sump high capacity filters. Make SURE you get allison filters, lots of knockoffs on the internet. motor coach technician has an excellent youtube video on this topic

https://youtu.be/Xa8gGfifKyE?si=B53ySRZejq1lBUu-

I watched the video.  It was very informative.  

Allison numbers appear to be 29558329??

Next question is can I mix transynd with whatever fluid is in there??

Yes, Transynd is backwards compatible. The only thing to note is that you are going to have a mix of fluids since you will not be getting all the fluid out. Generally they claim that after about 3 or 4 changes you should have full strength transynd for reference. Its not necessary to change it to get all the old fluid out unless you just want to.