With new federal regulations as well as compatibility issues with catalytic converters, the oils of yesteryear are gone. What this means is that the additives once found in high end oils are gone. These additives provided protection at the point when oil is the thinnest: the power stroke. While normal cars do not approach the same level of need that diesel and turbocharged applications require. Modern engines have less of a need for this added protection with today’s tighter tolerences to bearing surfaces.
ZDDP is the substance beign removed from oils. Modern car owners have the complaint that ZDDP damages catalytic converters over time. It is also toxic. Older car owners though need the added level of protection in their engines to keep bearing wear at a minimum.
Maintaining 6 cars with very picky oil requirements and related horror stories means I need to know about this stuff. The Saab T7 engines found in some 2000 to 2004 Saabs are notorius for engine sludge. There are a number of reasons why, but it most likely points to the catalytic converter being directly under the oil pan. The current mechanic suggestion is to use full synthetic 5W40 oil. Good luck finding that. After much reading, I went with Shell Rotella T Synthetic 0W40 found at Wal-Mart. Sicen all of the oils are changing due to regulations, you need to read the labels. Saab is standing by the Group III standard, although Group IV would by just as well if you can find it in the correct weight (Shell Rotella T Synthetic)
Check out the wikipedia page on Rotella: http://en.wikipedia.org/wiki/Shell_Rotella_T
