And here's one from Shell...I'm running Rotella 15-40 in mine.

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, 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
remained high. Products like Shell Rotella T Oils, which also carry API
S-series licenses for gasoline engines, have found favor for flat tappet
engines.

There has been a recent change to the API specification required for oils
used in 2007 big rig diesel engines to reduce particulate emissions, API
CJ-4. As a result, the zinc content in Shell Rotella T Multigrade Oils has
been reduced slightly from about 1400 ppm to about 1200 ppm. Even at these
slightly decreased levels, Shell Rotella T Multigrade Oils still have at
least 50% more of these anti-wear additives as most current gasoline-only
engine oils. These levels of zinc have historically offered good protection
in flat tappet applications.

Regards,
Edward Calcote
http://www.shell.com/us/lubricants/