I've always read that it was difficult to cool oil because it tends to cling to the surface.  Specifically, when the subject of cooling oil in the pan comes up, someone always points out that oil clings to the cooler pan surface, and gets a little thicker, allowing other hot oil to just flow over the top of that film of thicker oil.  In other words, the oil flows past without making any real contact with the relatively cool pan surface.  I always figured this same thing happened to some extent in the tubes of oil coolers as well.  So go ahead, shatter my mental reality and tell me this is all BS :-) 

Definitely not BS, so your cosmic egg can remain intact.  In the pan the oil is essentially static, so there is a major boundary layer conduction issue.  In any well designed oil cooler with narrow high-velocity channels with internal fins or turbulators - - Ah-h-h; hold it right there, the AC core doesn’t qualify.  Or are you using a well designed oil cooler?

But that is what I meant by a slightly better heat transfer coefficient with lower viscosity; less boundary layer effect.

In a couple weeks, we'll see if there's any real difference in temps. 

That will tell the tale.

Good luck with your fittings,

Al