As an previous owner of a compressor company, I used many, many types of oil. Amzoil was one of them. It failed in several compressors and cost thousand in repair. We switched to Mobil 1 and never had an oil related problem again.
The shop sponsored a VW race car. We used Pennsoil and had one failed engine after another. A Penns rep came to help us look into the problem. He said the Pennsoil was breaking down and he actually recommended we use Mobil 1. We did and ran the same engine for 2yrs with no problems.
I ran Mobil 1 in a GM car with a V-6. Had a head gasket fail at 72,000 mi, took the heads in to have the valves done and heads cleaned. The shop said the valves, etc were in mint condition. "What ever oil you're using, keep using it". It was Mobil 1.
Living in MN we did some very unscientific tests. We left several brands of synthetic oil outside in -20 degree temps. Mobil 1 was the one that poured out like it was 50 above.
Then we cooked them in a pan. All but the Mobil 1 turned dark brown.
We use Mobil 1 in all of our cars, Corvette, Acura TL, Suburban and now in my new '07 T4R and love it.
This is just my 2 cents worth and others may have other stories. These are just a few of mine.