FWIW, Mobil apparently has reformulated their 15W50 for 2007:
Why are you reintroducing Mobil 1 15W-50? Have there been any changes to the formulation, or is it identical to the previous product?
Mobil 1 15W-50 is being reintroduced based on popular demand. Mobil 1 15W-50 provides higher viscosity, designed to provide extra protection for performance vehicles and vehicles that operate in severe service, such as towing, hauling and racing. Additionally, Mobil 1 15W-50 contains higher levels of anti-wear (ZDDP or Zinc DialkylDithioPhosphate) that may be required for certain racing applications and camshaft designs. This is a new Mobil 1 15W-50 formulation and is not the same as the product that was marketed a few years ago.
from:
http://www.mobil1.com/USA-English/Mo...AQs.aspx#FAQs3