I don't have an oil-press gage on mine, but I've used Castrol "Syntec" automotive 10w40 with good results. It works *much* better (less vibration and smoother shifts) than the castrol moto-specific, semi-synth. "actevo" crap. My partner uses automotive Mobil1 in the FJ but we bought out all the local wal-marts of the old stock when they re-formulated the stuff to add friction-modifiers.
Keep in mind, "which brand/type of oil is best" is the kind of discussion that leads to holy-wars on message boards sometimes...
I ran syntec for about 10 years in my bike and five years in my Yukon. I recently found out that it is no longer a fully synthetic oil, hasn’t been for years. I am not saying that is not good but on the basis of (castrol's)lying I switched to Mobil 1. But I haven’t seen any difference. Check it out see what you think.
The Synthetic 5W50 oils tend to use friction modifiers in order to obtain such a wide viscosity gap. I'm sure Mobil 1 does, not certain about Castrol. It's these friction modifiers which are supposed to cause clutch slippage.
I don't think the 10W40 grades use the friction modifiers, so they may be a better grade of oil to use on bikes.