Castro Syntec 5w50?

no problems. Generally, if the engine isn't using oil then the viscosity isn't too thin. You cannot tell if the viscosity is too thick, except by reference to the engine manufacturer and the oil manufacturer.
The W number is irrelevant for normal operating temperatures. It merely indicates cold flow characteristics before the oil warms up.

0W 40 should protect as well as any 40 weight synthetic. The 0W number just means it passes the Winter or cold flow specs by such a margin they don't have a conventional oil that flows that well so can't give it a W number.

The W number isn't an actual viscosity indicator it is merely a cold flow measurement. The othe rnumber is the importan tone, how well the oil stands up at operating temperature. Very few modern engines benefit from higher than 40 weight.

approved oils someone posted a while back.

but my dealer used to use it on my S4 prior to 5W40 being readily available. I've used the Euro 0W30 that has the correct VW 502 rating that a MY'00 asks for.
Ask your dealer for the tech bulletin for approved oils.
Hugh