Here's a response from Castrol i got.

Thank you for contacting Castrol,

The latest API SM/ILSAC GF-4 category calls for reduced Zinc and Phosphorus
levels to allow extended catalyst life in current model vehicles. There
appear to be field issues associated with the SM/GF-4 oil's level of
antiwear in the classic car engines known as flat tappet cam engines. The
current late model passenger car engines are not flat tappet cam engines and
have no reported field issues related to the level of antiwear chemistry in
the SM/GF-4 oils.


Product Recommendations for Flat Tappet (Solid Lifter) Cam Engines:
Castrol Syntec 5W-40 (min Zn = 0.10 wt % = 1000 ppm)(full synthetic)
Castrol Syntec 20W-50 (Recent reformulation identified by "Recommended for
Classic Cars" text on back label) (min Zn = 0.12 = 1200 ppm)(full synthetic)
PREFERRED
Castrol TWS Motorsport 10W-60 (BMW dealerships) (min Zn = 0.10 = 1000
ppm)(full synthetic)
BMW Long Life 5W-30 (BMW dealerships)(min Zn = 0.095 = 950 ppm)(full
synthetic)

New Motorcycle Products, Expected to be Available Q2 2008:
Castrol 4T 10W-40 and 20W-50 (min Zn = 0.10 wt % = 1000 ppm)
Castrol Actevo X-Tra 10W-30, 10W-40, 20W-40 and 20W-50 (min Zn = 0.10 = 1000
ppm)
Castrol Actevo X-Tra Offroad 10W-40 and 20W-50 (min Zn = 0.10 = 1000 ppm)
Castrol Power RS GPS 10W-30, 10W-40 and 20W-50 (min Zn = 0.12 = 1200 ppm)
PREFERED
Castrol Power RS R4 10W-50 and 5W-40 (min Zn = 0.12 = 1200 ppm) PREFERED
Castrol Power RS V-Twin 20W-40 and 20W-50 (min Zn = 0.12 = 1200 ppm)
PREFERED

Existing Motorcycle Products, Will Be Transitioning Out:
Castrol Grand Prix 4-Stroke Motorcycle Oil 10W-40, 20W-50 (min Zn = 0.10 wt
% = 1000 ppm)
Castrol GO! 10W-40, 20W-50 Motorcycle Oil (min Zn = 0.10 = 1000 ppm)

Castrol Consumer Relations