If this is an original engine, never rebuilt, it was designed for 20W40 oil.
Most of this engine's life, the most popular oil, high performance or transportation duty, was 20W50.
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.