Ah, oil. Quite possibly the most hotly debated topic on this forum.
There's a ton of information (and argument) regarding Castrol Syntec in the forums on the Volkswagen TDI club webpage:
Apparently, Castrol Syntec was originally a "true" synthetic, but they later discreetly changed the formula without changing the packaging or the marketing.
Volkswagen dealerships use Syntec in their maintenance of the TDI engine, and it's what I bought from them when I started doing my own oil changes in our Beetle TDI. I plan to change to a different compression-rated synthetic (such as Mobil Delvac 1) at my next oil change on the Beetle, and plan to start using a NON-CASTROL compression-rated oil, such as Shell Rotella or Mobil Delvac, in my 240D whenI get the engine reassembled.
Keep in mind, of course, that Castrol Syntec is a fine and good motor oil. It has properties comparable to other synthetics on the market, and it will lubricate your engine just fine. However, I personally disagree with Castrol's marketing scheme, calling Syntec a "FULL SYNTHETIC" right on the bottle, when it indeed is not; therefore, I will NOT continue to use their product.
'83 240D "Steiner"
'00 New Beetle TDI
P.S. I predict that by this time tomorrow, this thread will have 3 pages of replies