I found some good info on Mobil's website, including this:
Question:
Oil Film Strength of Mobil 1 Compared with Royal Purple
Does Mobil 1 have less oil film strength than Royal Purple? I have seen many times the friction test of oils using the machine that test the oil film, test result shows that Royal Purple has more friction protection that Mobil 1 does, seeing these results makes me think that Mobil 1 is lacking some benefits, therefore Royal Purple has the better protection of all.
-- Guillermo Tristani, Miami, OR
Answer:
There are certain oils in the market today that use EP (extreme pressure) additives in their engine oil that are really designed for gear oils and not engine oils. Extreme pressure additives are typically not used in engine oils for a number of reasons but the most important is because they can cause engine corrosion over time. The rigs being used in these demonstrations are primarily designed for industrial applications like gear oils where extreme pressure is an important performance feature you need. These demonstration rigs have very little to do with modern engines and that is the reason that market leading oils in the industry perform poorly in these tests.
http://www.mobiloil.com/USA-English/..._Strength.aspx
Lots of other good answers on there too if you have time to read through it all.