I don't think you'd experienced much difference if you had stayed with the same viscosity as that right there would definitely without any doubt change the temp due to the fact the flow of one is different than the other.
The other point missed is that the antiwear/friction modifier additives between the two oils(syth/non synth) are not exactly the same, given that, can you actually contribute the difference because of the base oil or maybe the difference in the friction due to the additive differences?
My point is, there is no proof anywhere that in an engine, the synth base oil itself is what makes the difference in cases like yours since there is no constant with the ep/fm additives which can vary temp very easily.
This I can prove as I have done this on my timken machine.
When taking a oil for example like m1 supersyn, the friction it is producing while shearing the base stock gets so hot you cannot hold/ touch it BUT, when applying moly, the friction immediately drops and temp drops back down.
As of yet, I have not seen where base stocks actually carry off the heat any better in an engine because the design won't allow it.
One other point, we have a moly based mineral oil, have changed to the blend (combination of mineral and PAO), and in some cases have gone the reverse. In any case, there has been no difference in any temps that are even slightly noticeable. Now when I get a new customer to try the Schaeffers, and say they have been using an oil not as equipped to reduce friction due to less antiwear/fm additives, I have seen it drop. I have a couple of race cars that have told me the same as some individuals, but never, have I had anyone make any comments when switching between our blend and mineral.
The theory looks good, but in an engine, I don't agree with Molakules #'s IMHO.