DXIII, as has already been stated, Is the Tremec approved oil for WC T5 gearboxes.
IMO, Don't run additives as a general rule. But i have used G70 in T5's before, with no harm done. But this was before I learnt what I now know about oils.
If you want to learn some garbage, read on. Otherwise you can stop here.
Experimentally i recently did research for a fully synthetic replacement for DXIII that would be suitable for the T5. To help it with Heat and pressure stress from track use (abuse?)
I have since started using Pentrite ATF FS. https://www.penriteoil.com.au/products/atf-fs-full-syn#/
I purchased a 20L drum and use it (almost) wherever DXII or DXIII is specified. I have only done about 5,000km so far with the oil in my T5, and am having good results thus far. However it is way too early to say there wont be long term issues. So if you want the safe bet, go DXIII.
I decided to use Penrite ATF FS as an experement in my T5 since my 2nd-3rd shift under power was blocking. I suspected it was rooted and am expecting to replace (or rebuild) the box sometime. But after replacing the DXIII oil with ATF FS, shifting in all gears has improved and the shift up to 3rd does not block me under power anymore.
To expand on that, the oil i removed was stressed, having been used for a track day. So simply replacing the oil with fresh DXIII may have had the same effect. So don't go running around telling people that the ATF FS will fix shitty T5's.
To expand on that, Tremec themselves do TELL us in the T5 manual, to change the oil regularly. Especially in severe use conditions. Using a fully synthetic may improve longevity of the oil when compared to DXIII.
This is all just conjecture. And without spending a lot of time and money driving, switching oils and doing oil trend analysis, that is not going to change.
If someone wants to sponsor me i'll go ahead and get started? No? ... Damn