For those looking for a fairly good head to head comparison between Mobil 1 and Royal Purple, take a look at this recent independent lab test AMSOIL commissioned on most of the major motorcycle oils on the market:
AMSOIL Motorcycle Oil “White Paper” (1 MB pdf file)
Mobil 1 clearly outperformed Royal Purple in this test, as it does in most of the used oil analysis reports I've seen on the oil forums. For those that don't want do download and review that 1MB file, in the 40 weight category, Mobil 1 ranked 3 of 11 and Royal Purple ranked 7 of 11. In the 50 weight category, Mobil 1 ranked 2 of 15 and Royal Purple ranked 8 of 15.
In response to the comment/question about shear stability (e.g. a 0w40 shears down to a 0w30 or a 0w20) take a look at the shear stability test in this report on pages 9-10. Royal Purple sheared low out of grade in both 40 and 50 weight tests.
Here is a quote from the report on shear stability:
"The results point out significant differences between oils and their ability to retain their viscosity. Within the SAE 40 group, 36% of the oils dropped one viscosity grade to an SAE 30. Within the SAE 50 group, 40% dropped one grade to an SAE 40. Most of the oils losing a viscosity grade did so quickly, within the initial 30 cycles of shearing.
It should be noted that both high and low viscosity index oils exhibited significant amounts of shear and viscosity loss. Two of the oils with the highest viscosity index, Torco T-4SR in the SAE 40 group and Yamalube 4R in the SAE 50 group, had the largest drops in viscosity of all the oils in their respective groups. Torco T-4SR sheared to a SAE 30 and Yamalube sheared to a SAE 40. Valvoline 4-Stroke SAE 50 and Castrol V-Twin SAE 50 had a comparatively low viscosity index and they too lost significant viscosity, shearing down to SAE 40."
:cheers