if all O-ring are found to be in place, i second the 5w-40 option - or 0w-30. i have ELF Full Tech 0w-30 MB229.5 -
LINK
i live in Roanoke, VA and yesterday it was ~20 degrees when i started my 240 at 5:40am. she eagerly started and the oil pressure launches from 0 to 3 bar (with the ELF Full Tech 0w-30 compared to a 15w40).
not cheap though - but it should last a long long time.
here's an example of a TDI owner running ELF Full Tech for 19,600 miles with three lab analyses confirming optimal oil performance the whole way -
LINK
i pick mine up at
www.TDIparts.com
cheers!