Thanks Joel,  I'll plan on changing it in a year then.

Castrol told me to do a simple fluid change plus filters. This means I put in 7 gallons of Transynd instead of 9 gallons for a complete drain. Castrol says to change the Transynd again at 50,000 miles or 1 year . This 2nd change will mean I am running 99% Transynd. At this point Allison would warranty my transmission.

Actually you end up with a little over 75% Transynd after your first drain and fill change with new filters getting about 31 out of 40 quarts to drain per the Allison website with a 2 inch sump. I think I got 30 quarts in mine with my first change. Jeramy, if you have to drive far to pick up the fluid you may want to pick up 8 gallons to be sure.

The second change gets you to 95% Transynd which Allison considers acceptable for the extended change intervals. Allisons actual limit is 90% for the extended change intervals. At this point you still have about 2 quarts of earlier Dexron versions in the mix. Your third transmission fluid change would get you to the 99% Transynd. Fluid capacities are here for the 4000 series among others.

http://www.allisontransmission.....leToGet=SA5429EN.pdf

Jeramy, given the age of these transmissions, I do not think we would be under warranty, even with the Transynd change over but I am not sure of this. Maybe you could buy an extended warranty at this point? You mentioned Castrol... I think you would need something that is documented from Allison to be sure of the warranty policy. Hope this helps.

Later Ed

Transynd is compatible with Dexron.

Quoted from Richard And Babs Ames

Transynd is compatible with Dexron.

Right, it is totally compatible but a significant percentage of it in the fluid will degrade the "long lasting" properties of Transynd so you have to treat the transmission as if it were filled with Dextron and not Transynd.

Transynd is made by Castrol.  My percentage was wrong......it is 95% at 2nd fill.   My trans temp gauge dropped from 200 deg F to 170 deg F when I changed  to Transynd.

Since I'm not sure if my transmission is on its first or second fill of Transynd, I'm going to have an oil analysis done before having it replaced.  The analysis will be able to show if it needs replacing.  I'm going to use the low-cost kits available from these folks--http://www.jglubricantservices.com/  The owner, Tom Johnson, was a senior Allison engineer and describes himself as "the father of Transynd".  

Jeremy, In one of your posts you state that if you have 95% Transynd in your tranmission that Allison will warranty your transmission. Am I getting that correct? Change to Transynd and they will put a warranty on your 11 year old transmission? Marty

Hi Marty,
  I will try to verify this with Allison but I believe it to be true.  Castrol suggested this would be possible after the 2nd oil change. Maybe you can research  this for me as I am in Baja right now.  It would be good to know this wasnt hearsay !!
this from another forum  "  The Allison dealer gave me a form called an "Allison Transmission Extended Transmission Coverage Agreement Registration" Form SE0010 (8/04). What it amounts to is: you pay Allison $300, show that the truck already has Transynd in the tranny or pay them to flush/install Transynd and Allison will provide a 5 year warranty with no mileage limitation. In my case they inspected the transmission and confirmed that it has Transynd in it. I paid them $300 and they gave me the 5 year transmission warranty.
Perhaps this is what the Castrol rep. was referring to??  

Jeremy, I find that hard to believe. I have made myself a note and will try to get a hold of Allison to get an answer. I do know that in the trucking industry, if you specced out a new truck with synthetics in the diffentials and the transmission, the manufacturer warrantied them longer (from 100,000 to 500,000 miles). In all my years in trucking I never heard of any company offering a warranty for changing to synthetics on used running gear. I'll let you know what I find out from Allison.
Marty

Ok Marty,
I shall be very interested to hear what you can find out.    Jeremy

Jeremy, I just spoke with the service dept. at Pacific Power Products in Kent, Wa. 253-854-0505. If you put in a remanufactured Allison transmission in your older vehicle and choose to put Transynd fluid in it, you will get an extended warranty over just using Dexron III. The warranty is not for just changing to Transynd. Marty

Hi Marty,
  thank you very much for clarifying this . Some service centers give out wrong information and this was one of those times.    I guess it is good news for those of us who have to install a rebuilt transmission.