Here is a response from a Shell Oil Engineer from April 2008
Subject: RE: Rotella Ask Our Expert: Oil and Fuel Additives
Richard,
With the introduction a couple of years ago of ILSAC specification GF-4 and
API specification SM for gasoline-only engine oils, the amount of
phosphorous in the oil had to be reduced to prolong the life of catalytic
converters. Zinc-based anti-wear compounds, like ZDDP, which also contain
phosphorous, were reduced as well. These zinc compounds work well to
protect flat tappet engines from excessive wear, so the modern gasoline-only
engine oils are not as effective in this regard as they once were.
Diesel engine oils don't suffer from the same requirements to protect
catalytic converters, so the amount of zinc-based anti-wear additives
remains high. Products like Shell Rotella T Oils, which also carry API
S-series licenses for gasoline engines, have found favor for flat tappet
engines.
Shell Rotella T Oils contain about 1200 ppm zinc. That's at least 50% more
of these anti-wear additives as most current gasoline-only engine oils, and
about the same amount of zinc as API SJ-rated oils had in 2000. These
levels of zinc have historically offered good protection for flat tappet
applications.
There are no proposed specification changes for diesel oils that would
significantly impact the zinc content of Shell Rotella T oils over the next
2-3 years.
Thank you for your interest in Shell Rotella Products!
Regards,
Edward Calcote
Staff Chemist, Shell Lubricants US Technical Information Center
http://www.rotella.com/