Hi, still experimenting with oils for my Pontiac 400. I just put some of this Mystik 15w50. It is the new synthetic blend which is still rated CI-4/SL. The 15w40 has gone to the new CJ-4. I contacted Citgo and they said it has 1300 ppm of zinc. Is this a good oil for a 10:1 400 with XE274 Comp Cam with 995 Valve Springs(heavy)?