[David R Leifheit]

'tis funny.
From what I can find on Castrol's site, they no longer manufacture "Syntec" it is now called "Edge". They replaced the Zinc with Titanium for better protection at higher heat. Some of the old car forums have been discussing the Zinc levels in Castrol Syntec 5-50....

What I am seeing reading all these oil company sites, and commentaries, is that the government regulations restricting the use of zinc has created the demand for titanium to replace it.

Actually quite interesting reads.

Hemmings has an article on which zinc formulations to look for, but it is out of date (2012). Apparently though Castrol was one of the first to use zinc, and as the amounts allowed were decreased they turned to other additives currently using titanium for the same effect. With the added bonus that apparently the titanium does not need the phosphorous, so the carbon buildup due to phosphorous in the oil isn't happening.