Jfsram

A synthetic differential fluid in the same grade WILL offer you gains. Problem is they are small and without test equipment you will not notice it.

I've seen enough magazine test, back to back, after swapping to a synthetic fluid, gained.

It's gauranteed power from the engine too. Yes, seen enough dyno sessions to know that too. After witnessing this do I run synthetic in all my cars. Hell no. It's too expensive for a minute gain.
I'm specifically refering to engine oil here, since it's changed often. A extra cost of differential oil is not a big deal, since you don't drain it often.

I also work with industrial machinery which is meticulously logged for maintenance, power drain, efficiency, heat and life. Synthetic has also proved itself there for me too but that's a no brainer since I'm not footing the bill.