In some respects, Castrol got themselves into the mess of multiple similar products.
First there was dino white bottle GTX. A great oil which I used in great results! It was generally considered, in 20W50, a "motorcycle oil" brand in many areas of the USA. It had GREAT heat stability in hyper-overheated engines, even noted for "saving them", which I happened to prove one night, NOT on purpose!
When Castrol went against Mobil1 in the synthetic products, in the USA, that was "black bottle" Edge. A semi-synthetic came later. This was the gray bottle "Syntec"
Magnatec was an oil brand name in Europe back then, probably not completely related to the Magnatec we have in the USA? Castrol had several product names in Europe that weren't in the USA back then, as now. The Castrol website denotes "GTX Magnatec".
The gold bottle Edge came later. It includes the consumer "Extended Performance" product and the import dealers have a gold bottle Edge with an "OE Performance" name on the bottle. And now titanium has been added to most Edge products, it seems.
And in all of this confusion, "German Castrol" surfaced in the USA! Check the related threads in these forums! IT was only available at AutoZone auto supply stores in the USA and ONE viscosity level. The bottle originally noted "Made in Germany", but later versions did not. This was well before any other oil brands started to use "Euro" in their oil names.
I used the Syntec semi-syn Castrol in my flat tappet engine until it became an SM oil, then I went to Rotella 5W40 for the added zinc levels. For a good while, the SL oils for the import engines had 1000ppm of zinc (as their spec amount, even Mobil1), but I believe they've all been superceded to SN status.
I do have fond memories of using Castrol and how much better it seemed to perform than other oils I'd been using (1970s-mid 1980s). I DO wish they'd get their product line better separated as it's somewhat confusing with several levels of seeming "the same" oils to choose from. GTX is their dino line, Magnatec is semi-syn, but then it gets flaky as they seem to desire to use all of their prior names on the synthetic products. Also notice that some of the prior oil product names are now "technology" names (i.e. "With Syntec Technology"), all of which seem to be unexplained. Semi-syn "technology" in a syn product?
That's how I recall their product progression.