VW of America specifies CD-rated oil for the diesels, in the owner's manual. This specification was obsolete at the time of manufacture - read the next section to see what the ratings mean, but many revisions (latest is CI-4 I think) have superseded it completely. It's even been said that the CD rating is no longer allowed to be used, it's so out-of-date. VW has specified to Castrol and to me that the TDI requires CG-4. But be aware that VW dealers may not use properly-rated oil in your car when they service it! Looking at the API ratings, though, it's clear exactly why CG-4 is appropriate for this application. Both Mobil and API refer to the high-soot tolerance of CG-4, while several oil companies are mentioning why SJ is now getting further away from the diesel requirements. One dealer was using Castrol GTX (not diesel-rated) and the other dealer used Quaker State (also not diesel-rated).
Per a nearby dealer, VW specified that TDI engines should use 5W30 for wintertime use, to avoid engine warnings when the cylinder head takes too long to get oil pressure. This is in agreement with the manual, which allws use of 5W30 (only in an Energy Conserving grade) during conditions of less than 100°F.
Finally, VW straightened it out by specifying Castrol synthetic 5W40, which is CF rated. But for a while, it was a real mess.
What do the API ratings mean? For starts, the "C" specification is for diesel (compression ignition) and the "S" series is for gasoline (spark ignition). Within each category is a series of specifications like CC, CD, CE, CF, CG, CH, SG, SH, SJ, etc. Look at the API website for better info. If you just want the short version, this was from Mobil's website:
Here is a short description of the diesel engine categories, starting with the most current.
CH-4 is a proposed category at this time, which we anticipate being finalized during the first half of 1999. CH-4 is being developed to address engine design changes required to meet new 1998 EPA emission standards.
CG-4 was developed for oils used in 1994 and later, low-emission, high-speed, four-stroke cycle diesel engines. It is also typically used for diesel engines using low-sulfur diesel fuel. Newer engines that are used on-highway or off-highway applications may call for CG-4 quality oils. One of the main advances in CG-4 versus previous quality levels is the ability of the oil to handle high levels of soot that is generated by some of these low-emission engines.
CF-4 was established in 1991 to specify oil quality for severe- duty, low-emission, four-stroke diesel engines produced in 1991 or later. Oils meeting this category offered improved control of oil consumption and piston deposits versus previous quality levels. CF-4 has essentially replaced the CE quality rating.
CF-2 was established at the same time to specify oil quality for severe duty two-stroke diesel engines. CF-2 has essentially replaced CD II.
CF was established for service typical of off-road, heavy-duty diesel engines and engines operating on higher sulfur fuels. The CF quality level essentially replaced the CD level.
CE was established in 1988 to specify oils for severe-duty diesel engines with turbochargers. This rating is essentially obsolete and oils meeting CF-4 are now used in applications calling for CE oils.
At the time of purchase, I found the following oils to be certified for diesel use. All were cross-checked with the API listings. This is now out of date, but the links might be useful so I'll leave it here:
(websites make references to other ratings, but API does not list them)
per Barbara at Customer service, (973) 633-2200:
SynTec Full Synthetic (CF)
RX Super (CG-4)
SynTec Blend in 15W40 (CG-4)
GTX is NOT for diesels
Delo-400 15W40 (CH-4)
(Texaco) per http://www.texaco.com/tlc/products/havoloroilsn.htm
Havoline Formula3 Synthetic, SAE 5W-30, 10W-30 and 5W-40 (CF, EC)
Mobil 1 (CF)
Delvac 1 and Delvac 1300 (CG-4)
per telecon and email with their tech services:
Long-Life (CH-4, CG-4), in 30, 40, 50 and 15W-40.
Supreme Duty (CH-4, CG-4) in 30, 40, 50 or 10W-30.
"Formulations in "SJ" motor oils have changed to meet current demands for gasoline engines. We no longer recommend oil with a single SH or SJ API rating for diesels."
FCI grades are CG-4 per their catalog
Rotella T ?
All-Fleet Plus (CF-4)
Racing Formula (CD)