My uncle uses Castrol 40wt in his race engines when the temp is not a factor and Castrol Tection Extra 15W40 if temps get a little cooler (he's in south Alabama so temps are a bit warmer) He just recently bought a lot of it at a bankrupt parts store auction and will make you a deal on the stuff if you are near him. The Tection is loaded with zinc (it's the earlier spec) he bought it just because he knew it was high zinc content oil and its getting difficult to get.
Below is from a Castrol rep response.
"HD 30 has relatively high level of Zinc and also has a treat of
Molybdenum. I would not expect cam issues with this product unless valve
spring pressures are so high that a higher viscosity grade is needed.
Syntec 5W-50 has lower Zinc level than HD 30 and slightly lower Moly
level and is more typical of a current GF-4 product.
Assuming that an API SG oil was fully backwards compatible with OEM
muscle cars and mild to moderate high performance after-market engine
work, and, if a "typical" SG PCO oil is assumed to have a Zinc level of
0.10 to 0.12 weight percent; then, the following Castrol products have a
Zinc level that is HIGHER than 0.12 weight percent:
* GTX 20W-50 (SL,SM) (also has a Moly treat)
* Castrol GTX Diesel 15W-40 (CI4,CH4,CG4,CF4,CF,SL)
* Castrol GTX High Mileage 20W-50 (SL,SM)
* Castrol HD 30 (SL,SM)
* Castrol HD 40 (SL,SM)
* Castrol Syntec Blend Truck 15W-40
(CI4,CH4,CG4,CF4,CF,SL)(Semi-synthetic)
* Castrol Tection Extra 15W-40 (CI4Plus, CI4,CH4,CG4,CF4,SL)
* Castrol Hypuron S 15W-40 (CI4Plus,CH4,CG4,SL)(Semi-synthetic)
Note that HD 30, HD 40, and GTX High Mileage also have a Molybdenum
treat which offers friction modification on top of the protective Zinc
film."
