I'm using the Syntec 5W-40 that says it's made from imported and domestic components...the newest version...NOT Belgian Castrol. I did some high temperature testing on it along with Gold German Castrol, heating both of them up to over 300 degrees for several hours and when they cooled, compared the viscosity of each, volume lost to evaporation and whether any deposits formed on the anodized aluminum cups both oils were in. I screwed up the volume measurements but could tell that the difference was very minor. Neither oil left any deposits at all on the cups. And the viscosity measurement technique was very crude because I was in a hurry, but there didn't seem to be much change from the virgin viscosities. I then heated up the same abused 5W-40 Syntec test oil with a blow torch to obscene temps for 1 minute (didn't measure temp but based on previous tests it likely reached at least 450 degrees). I was careful to not allow it to catch on fire (the oil vapor above the oil was too "rich" to burn). It turned dark like all oils do in these conditions but no deposits were left on the bottom of the cup which has occured with several others synthetics I've done this same thing to. Overall, the new Syntec 5W-40 seemed to fair very well considering that I did much more thorough/accurate tests on the Gold GC during earlier tests and it was very robust compared to other top synthetics. I was unusually sloppy with these tests and would like to have given you more accurate test results but I did these tests to give me a quick answer as to whether I should put it in my much spoiled VW 1.8T engine. Perhaps I'll retest and be more thorough. I've been wanting to test Syntec 5W-40 alongside M1 5W-40 T&SUV in a battle of the titans!
P.S. My point is that we have nothing to worry about in the high temperature robustness of the basestocks and additive package of at least the new Syntec 5W-40 and I'd expect the same from the Belgian version too.