According to this product page:
https://epson.com/For-Work/Printers/Inkjet/WorkForce-ST-C8000-Color-MFP-Supertank-Printer/p/C11CH71202it's "DURABriteĀ® EcoFitĀ® ink"
Keep in mind that this is a 4 color inkset--
I have to say though, we use one of the first models as an office printer, and it's been fantastic for printing documents (the ET-16500). Even without airprint, and with the relatively low quality of inkjet on plain paper, it's super fast to start up, and we've gone through tens and tens of thousands of pages and spent next to nothing on ink...
I've been wanting to experiment with putting other inks in it's tanks since we first got it (this model came with pigment black and dye CMY). It's definitely not for those looking for high quality photo prints, but for blasting out office documents, or even printing instruction manuals it has been fabulous.
I've seen quite a few chinese solvent and sublimation printers coming out that use heads from the Workforce series-- pure speculation, but it looks to me as if they've gotten the manufacturing cost down so low on the previous generation head tech (maybe from the 11880s and 9900s?) that they are able to make 4 and 6 color versions easily...