In the normal USA market "Castrol GTX" has been around since the early 1970s. The name has more consumer awareness than the "RX Super" name--in the USA. Other venues might be different. Unless there have been some reverse-engineered analysis of the 15W-40 GTX Diesel and the RX Super, to claim they are "the same" (although they might seem to be) could be incorrect.
I would classify the Tection to be for "over-the-road" style diesels, but which could also be used in older vehicles that might need the extra ZDDP it might have. The Castrol website, in some cases, has some specs on various additives in the oil (incl. ZDDP)--for oils which have an "SL" and "SM" rating. It takes more than some extra ZDDP to make up the OTR orientated additive package. In any case, look for the OEM approvals to see if either GTX Diesel or Tection Plus are what you might need for your more modern vehicles.
Enjoy!
CBODY67