In 2 words, 505.01 is a mixture of additives and anti wear stuff. Many known manufacturer have it like Castrol TXT from dealer and the following companies.
http://www.my-gti.com/2532/volkswagen-oil-standard-505-01-approved-oils
I used 15W40 in my 1985, 1989 and 1991 VW Diesels. Now have...