Hard for me to describe, but I could tell that the engine had less of a load on it after changing over to synthetic differential fluid. It was slightly less sound coming from the engine compartment and the acceleration was smoother. Gas mileage was slightly better. It was roughly 1-2mpg depending on my speed. I used Amsoil Severe Gear 75w-90. RP should behave similarly.

 

Can't say that I noticed a difference after swapping to synthetic. I can say that after 100+ miles, the fluid was dark and probably needed to be changed. I'm glad that I swapped to synthetic just because! Lot's of noise about longer life of the parts and no negatives found about the swap. Supposedly it lubricates better with less friction. I might change it again if I get into some really deep water or if I get bored just to see what it looks like after some miles.