I would imagine it's a factory designated product label borrowed from GT (the Gran Tourisimo) label that sporting versions of car models was created for. If you had a Ford Saloon with the GT badge it would be one with a souped up engine and possibly twin-carbs and beefed up suspension in the guise of a Rally Car but only the street version of same.
So I would bet that Castrol called it GTX to make it seems Xtra and specially designed for the GT vehicle.
In fact it turned out to be an excellent all round oil for just about any 4-stroke engine petrol driven (gasoline).
Nowadays Castrol do a GP oil which is even more advanced Castrol GPX or Grand Prix Xtra (see where they're coming from?) GPX is one of the best performing oils for high powered high revving motorcycle engines too.
By the way I have no connection with Castrol company - I'm just a long term user of their oils that seem to be good value if anything in the oil market is ever good value again.
I hope that answers your question, but it's only my guess. Google search will probably prove me wrong but I bet it's out there already.
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Dartmoor Creeper
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