HAMB member Blues4U states that he is a lubrication engineer and his posts confirm that he is quite knowledgeable about engine oils.

One of the points he makes is that modern HDEO is very suitable for flat-tappet gasoline engines and that these oils do not require ZDDP supplements. This is in agreement with GM TechLink's Engine Oil Myths.

The Delo and Rotella appear to be excellent flat-tappet engine lubricants even with the much lower phosphorus concentrations than the 1400+ ppm that many people believe are crucial. I also think that it is not beneficial to use an oil that is thicker than necessary. See Engine Wear.