I don't know, I've ran penzoil in everything ive ever had and had plenty of occasions when doin builds to pull engines apart and the only thing i can absolutely say for my personal experience is don't run quaker state dino , 65k mi in a I-6 4.0 in a wrangler, guy never missed a oil change and I had to use a friggin spoon to scoop the sludge off the head which had taken the shape of the inside of the valve cover

(API testing is what I think you meant there JeepinJarhead.)

API testing of motor oils is only to meet a minimum standard. Funny thing is there are recycled motor oils on the market that pass the minimum standards the same as the rest of the API oils. Does that mean they are as good?

Amsoil uses formulations that are not API compliant but they are engineered to meet the needs of an application, not some across the board API standard. Amsoil engineers oil to perform better than the competition in most regards w/out being tied to API's requirements.
Amsoil guarantees their oil regardless of API and will warranty any oil related failures, (which are extremely rare for any oil anyway) And then file suit against any MFG or dealer that denied claims, forcing Them to prove that a failure was oil related. This goes for Amsoils API certed and non-certed oils. To date, since 1972, Amsoil has not lost a case related to their warranty. (same goes for all their products, not just engine oil)

oil arguments, its all a pissin' match really. use whatever turns your crank. chances of oil related failure are small regardless of brand used. However, if you want peace of mind using one of the best oils, want to take advantage of extended service intervals, great filtration products, etc, etc. then use Amsoil. If not then don't.

One of our 4.0's after 170k. Its had some Pennzoil but mostly Valvoline and Valvoline Max Life.