Yep most Castrol Magnatec oils, which were semi-synthetic, have been upgraded to full synthetic. While most mineral GTX oils have been upgraded to semi-synthetic GTX UltraClean. Most, but a few exceptions exist.
Here in Oz they have released Castrol GTX UltraClean 15W-40 semi-synthetic which is API SN/CF and Euro ACEA A3/B3. It's almost always on half price sale at something like $20 for 5L, that's a smoking hot price for Australia where vanilla M1 5W30 costs $100 for a 5L jug.
Given the price, yes I have used it, and yes I have a few extra jugs lined up in my stash. It works fine and I typically run it 8,000 to 10,000 km (5k to 6k miles) without concern.
Sorry I can't comment about the thinner stuff. But it's a name brand oil that's rated API SN / SN-Plus and ILSAC GF-5, so if that is what you require, it should do the job.
BTW Yes, we are allowed to run thick oils in everything in Oz, it's in the constitution or something.