Here is a table of Mobil 1 products and their zinc and phosphorus levels. Looks like Mobil 1 V-Twin 20W-50 is loaded with both and is of the proper viscosity.
http://www.mobiloil.com/USA-En..._1_Product_Guide.pdf
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.