Of the three Transtar ATF products listed, I would guess your mechanic is using the Mercon labeled.
Transtar does not manufacture or blend their own lubricants. They purchase bulk at a discount and re-label it with their brand. The question becomes are they just buying the cheapest fluid on the market or are they buying the bulk fluid that has the best analysis? I have read they are buying based on least expensive and offers the highest markup.
Popular brands on 4R100 forums are typically Mobil-1 and Valvoline. These companies manufacture and blend their own products and have high user ratings. Ofcourse, another top quality option is the Motocraft brand of Mercon-V. Ford does not manufacture this ATF; however, they insist it meets very specific properties that are best suited for their transmissions.
Do you intend to do work on your own 7.3 or find it more advantageous to have a shop do the work? No right or wrong answer.
I mean no disrespect towards your mechanic choosing Transtar. I am guessing he services many types of Transmissions and finds the Transtar does the job.
But our 4R100 Transmission is around 20 years old and about $4K to replace. For that reason, I would suggest budgeting for another pan drain & fill within the next 5,000 miles and using one of the following ATF fluids:
1) Genuine Motorcraft Mercon-V (corrected)
2) Mobil-1 Multi-Vehicle ATF
3) Valvoline Dex/Merc (Blue Jug)
4) Valvoline Max-Life ATF (least expensive option).
I would also perform Mark Kovalsky’s ATF Flush Procedure if doing the work myself (or does he go by Mike nowadays? 😉).
If taking it to a mechanic, then getting on RT’s rotation schedule would be good.
My own preference if I relied on a Mechanic would be to find a shop that specializes specifically in Ford Powerstrokes. They have learned the unique peculiarities or requirements of our Vintage.